Refactor webhooks package

[adriansmares]

Summary

References https://github.com/TheThingsIndustries/lorawan-stack/issues/4163
References https://github.com/TheThingsIndustries/lorawan-stack/pull/4183

This PR backports the changes related to webhook durability in order to lower the reviewable diff in enterprise.

Changes

• Reserve API fields to be used by webhook queueing.
• Decompose the web package.
  • It was a pretty huge monolith. We've added many features over time in this package, while also attaching them quite heavily to the webhooks struct and so forth.
  • webhooks.go got split a bit, specifically functionality which wasn't really part of the webhooks object was split into smaller components in order to allow reusability and more focused testing.
  • The sinks have been torn out of web and got their own dedicated package.
  • The context utilities have been extracted into an internal package and got compressed into a single context.WithValue in order to lower the number of allocations a bit. Just chaining context.WithValue 3 times when we always do it in the same order and with the same structure is just wasteful.
• Clarify the semantics on who generates a failure / a success when it comes to events and metrics with respect to webhooks.

Testing

This is strictly a refactoring without new functionality changes. Most of the code is just strategically moved around in order to decouple it. No manual testing is required, and the existing unit tests should suffice.

Regressions

N/A.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.