Feat: KV store on RequestContext class

[trevorriles]

Add a kv field to the baseplate RequestContext object to allow passing arbitrary data.

This can be used to explicitly pass opentelemetry context across processes, for example with our internal graphql codebase we can do things like:

Attach to context in kv store

trace.context_api.attach(context.kv["otel"])

Store span in context:

trace.set_span_in_context(span, context.kv["otel"])

Of course this can be used for other non-tracing purposes as well.

[trevorriles]

@chriskuehl I'm going to close this for now. I put this together to solve a specific issue I had within graphql. I'm going to see if I can solve it with out introducing functionality that might increase confusion within bp.py core.

I may be back with something similar as I begin to work with the graphql team, but we don't need this in the upcoming release.