[Proposal] Break out and expand IApiCallDetails

[stevejgordon]

Today we use IApiCallDetails as a bucket of diagnostic and HTTP metadata about a request/response. While this is useful, it also causes allocations for each request and mixes functionality.

It might be useful to split this such that IApiCallDetails becomes lighter and focused on HTTP specific fields such as status code, URI, mime-type etc. These would be things we likely want to store per response. If small enough, this may also benefit from becoming a struct in the default implementation.

We could then move diagnostic fields into their own type, IApiDiagnosticDetails such that we can have an opt-out mechanism that avoids the allocation of the instance at all. Additionally, it separates the diagnostics properties from the more commonly used HTTP-related data.

We'd need to do some profiling to compare the effect of such a split as the additional field increasing the size of ITransportResponse.