Override individual BCD keys

[ddbeck]

We need a way to be able to override the status results for a single BCD key at a time. See https://github.com/web-platform-dx/web-features/pull/1376/files#r1675836543 for an example. A headline status can conflict with the generated per-key statuses and that's pretty undesirable.

I suspect this needs to happen at the compute-baseline level. So we'd probably need a few pieces:

• A new overrides option to Compat(), so we can substitute per-key data at the time the initial compat tree is constructed.
• A way to record those overrides (e.g., some YAML or .ts files with some kind of well-defined exports interface). It'd be ideal if we included the data were were overriding, so that if the overridden data ever changes, we're forced to review that on a BCD upgrade.
• Tests (e.g., confirm that unmodified data remains unmodified).