Refactor filter configuration to make the different test server filters independent

[dubious90]

This is a followup to #852. In that change, we duplicated ResponseOptions onto separate DynamicDelayConfiguration and TimeTrackingConfiguration protos. However, this resulted in many fields on those protos that were not relevant to each filter. In this PR, we make it so that only the relevant fields exist on each proto.

Resulting changes:

1. Moved DynamicDelayConfiguration and TimeTrackingConfiguration into their own files, and moved their fields into those protos directly.
2. In order to make request headers still work, there is now manual logic for taking in a ResponseOptions proto via json and cherrypicking the fields relevant to each filter.
3. The generic method computeEffectiveConfiguration was moved to each filter's anonymous namespace.
4. All configs, tests, and markdown files updated to use the new protos.
5. Some tests have been removed that are no longer necessary, or never provided additional coverage, including some that are less feasible now.
   • HeaderMerge was removed, but everything it tested for was moved into new unit tests of MergeJsonConfig.

[mum4k]

@qqustc please review and assign back to me once done.

[mum4k]

Can we as part of this PR add a CHANGELOG.md file and document the breaking change? Specifically we should call out (with an example) what broke and how users are expected to modify their configs to make it work again.

Looks like we introduced two breaks, so we should document both:

1. A break when we moved onto the experimental_response_options fields.
2. A break where we removed the experimental_response_options fields.

[dubious90]

@mum4k Believe I have addressed all your feedback now.

[mum4k]

Thank you for adding the documentation and tags. Sending a round of review on the docs.

[dubious90]

Addressed feedback.

[mum4k]

Thank you.