feat(types): add Response context

[lcian]

Adds the Reponse context, which can be attached to events.

The implementation is adapted directly from the one in Relay, minus the inferred_content_type, which as documented here can technically be sent by SDKs but will always be overridden by Relay.

Part of #873

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.13%. Comparing base (4c4fc9d) to head (1de6b99).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

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- Coverage   73.14%   73.13%   -0.01%     
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  Files          64       64              
  Lines        7365     7366       +1     
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  Hits         5387     5387              
- Misses       1978     1979       +1     

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[szokeasaurusrex]

Nice! Just a few questions:

• How is this new type supposed to be used? Does not look like we are using it in the SDK, is it something that users are just supposed to know how to set? Or, does the into_context! stuff take care of setting it?
• Do we anticipate ever needing to add more fields to this struct?

[lcian]

• How is this new type supposed to be used? Does not look like we are using it in the SDK, is it something that users are just supposed to know how to set? Or, does the into_context! stuff take care of setting it?

I'll make a PR on top of this to use this in the Actix Web integration.
We already capture most of this data using the Request Interface but we're missing the status code.
This way, not only we will report the status code to Sentry, but most importantly a user will be able to filter an error in before_send based on the status code.

[szokeasaurusrex]

Lgtm, maybe you could add an example to the changelog, though!