instrumentation-http: Introduce a hook to allow customizing the Context

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Is your feature request related to a problem? Please describe.

I'm building a caching library with deep integrations into the open telemetry API, and I would like access to additional information about user request when handling data requests.

It seems to me like the Context object is the perfect mechanism for this, however there's no way to add custom properties to the context that's initialised for each request in instrumentation-http.

Describe the solution you'd like

Introduction of a new hook to allow customising the context with additional attributes.

Describe alternatives you've considered

N/A

Additional context

N/A