Add support for skip, skip_from, and start_from for flows

[jlantz]

This PR adds new options when defining and running flows to skip steps.

In cci:

$ cci flow info ci_feature_2gp
An update to CumulusCI is available. To install the update, run this command: pip install --upgrade cumulusci
Description: Install as a managed 2gp package and run Apex tests. Intended for use after build_feature_test_package.

Flow Steps
1) flow: install_2gp_commit [from current folder]
    1) task: github_package_data
       options:
           context: $project_config.project__git__2gp_context
    2) flow: dependencies
        1) task: update_dependencies
           options:
               resolution_strategy: commit_status
               dependencies: ^^github_package_data.dependencies
        2) task: deploy_pre
           options:
               path: unpackaged/pre
    3) task: install_managed
       options:
           version: ^^github_package_data.version_id
2) flow: config_apextest
    1) task: deploy_post
       options:
           path: unpackaged/post
    2) task: update_admin_profile
3) task: run_tests

$ cci flow info ci_feature_2gp --skip deploy_post,run_tests
Description: Install as a managed 2gp package and run Apex tests. Intended for use after build_feature_test_package.

Flow Steps
1) flow: install_2gp_commit [from current folder]
    1) task: github_package_data
       options:
           context: $project_config.project__git__2gp_context
    2) flow: dependencies
        1) task: update_dependencies
           options:
               resolution_strategy: commit_status
               dependencies: ^^github_package_data.dependencies
        2) task: deploy_pre
           options:
               path: unpackaged/pre
    3) task: install_managed
       options:
           version: ^^github_package_data.version_id
2) flow: config_apextest [skip]
    1) task: deploy_post [skip]
    2) task: update_admin_profile
3) task: run_tests [skip]

$ cci flow info ci_feature_2gp --skip-from config_apextest
Description: Install as a managed 2gp package and run Apex tests. Intended for use after build_feature_test_package.

Flow Steps
1) flow: install_2gp_commit [from current folder]
    1) task: github_package_data
       options:
           context: $project_config.project__git__2gp_context
    2) flow: dependencies
        1) task: update_dependencies
           options:
               resolution_strategy: commit_status
               dependencies: ^^github_package_data.dependencies
        2) task: deploy_pre
           options:
               path: unpackaged/pre
    3) task: install_managed
       options:
           version: ^^github_package_data.version_id
2) task: config_apextest [skip]
3) task: run_tests [skip]

$ cci flow info ci_feature_2gp --start-from config_apextest
Description: Install as a managed 2gp package and run Apex tests. Intended for use after build_feature_test_package.

Flow Steps
1) task: install_2gp_commit [from current folder] [skip]
2) flow: config_apextest
    1) task: deploy_post
       options:
           path: unpackaged/post
    2) task: update_admin_profile
3) task: run_tests

And in cumulusci.yml flow definitions:

flows:
     ci_feature_2gp_no_test:
         description: Run ci_feature_2gp without tests
         options:
             steps:
                 1:
                     flow: ci_feature_2gp
                     skip:
                         - run_tests

skip_from and start_from also work in yaml.

Still needs test coverage but otherwise should be a backwards compatible change that unlocks a lot of potential for restructuring CI/CD workflows.

[salesforce-cla]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Jason Lantz <j***@j***.localdomain>. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, sign the Salesforce Inc. Contributor License Agreement and this Pull Request will be revalidated.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @agaudoncr to sign the Salesforce Inc. Contributor License Agreement.

[agaudoncr]

I got this error when trying to sign it Could not sign the CLA, please contact: ***@***.*** Could not sign the CLA, please contact: ***@***.*** Could not sign the CLA, please contact: ***@***.*** Could not sign the CLA, please contact: ***@***.*** Message ID: ***@***.***>

…

[jlantz]

Oops, need to rebase this branch, closing :)