fix: make broadcaster sink interruptable

What does this MR do?

Related to #1349 (closed)

This is one of the MRs/changes required to address the problem outlined here and to enable graceful shutdown of registry which is part of #1349 (closed) (namely the implement proper shutdown for for Notifications-related goroutines).

I tried to be as much backwards-compatible as possible and to preserve the current behaviour while making the changes, even if it meant slightly more complicated code than it coulde. The new config parameter is kind of a compromise between "wait indefinitely" behaviour we have now and broadcaster being interruptible. We do not have reliable delivery of notifications ATM, so timing out after receiving SIGINT and droppping notifications is OK. What is more - k8s issues SIGKILL if registry does not terminate in time after SIGKILL so these notifications would be lost anyway.

NOTE: I rely heavily on go channels and goroutines that communicate as this is the idiomatic way in Golang to implement synchronization in an interruptible fashion.

Author checklist

• Assign one of conventiona-commit prefixes to the MR.
  • fix: Indicates a bug fix, triggers a patch release.
  • feat: Signals the introduction of a new feature, triggers a minor release.
  • perf: Focuses on performance improvements that don't introduce new features or fix bugs, triggers a patch release.
  • docs: Updates or changes to documentation. Does not trigger a release.
  • style: Changes that do not affect the code's functionality. Does not trigger a release.
  • refactor: Modifications to the code that do not fix bugs or add features but improve code structure or readability. Does not trigger a release.
  • test: Changes related to adding or modifying tests. Does not trigger a release.
  • chore: Routine tasks that don't affect the application, such as updating build processes, package manager configs, etc. Does not trigger a release.
  • build: Changes that affect the build system or external dependencies. May trigger a release.
  • ci: Modifications to continuous integration configuration files and scripts. Does not trigger a release.
  • revert: Reverts a previous commit. It could result in a patch, minor, or major release.
• Feature flags
  • This change does not require a feature flag
  • Added feature flag: ( Add the Feature flag tracking issue link here )
• Unit-tests
  • Unit-tests are not required
  • I added unit tests
• Documentation:
  • Documentation is not required
  • I added documentation
• database changes including schema migrations:
  • Change does not introduce database changes
  • MR includes DB chagnes
    • Do not include code that depends on the schema migrations in the same commit. Split the MR into two or more.
    • Manually run up and down migrations in a postgres.ai production database clone and post a screenshot of the result here.
    • If adding new queries, extract a query plan from postgres.ai and post the link here. If changing existing queries, also extract a query plan for the current version for comparison.
      • I do not have access to postgres.ai and have made a comment on this MR asking for these to be run on my behalf.
• Ensured this change is safe to deploy to individual stages in the same environment (cny -> prod). State-related changes can be troublesome due to having parts of the fleet processing (possibly related) requests in different ways.
• If the change contains a breaking change, apply the breaking change label.
• If the change is considered high risk, apply the label high-risk-change
• Changes cannot be rolled back
  • Change can be safelly rolled back
  • Change can't be safelly rolled back
    • Apply the label cannot-rollback.
    • Add a section to the MR description that includes the following details:
      • The reasoning behind why a release containing the presented MR can not be rolled back (e.g. schema migrations or changes to the FS structure)
      • Detailed steps to revert/disable a feature introduced by the same change where a migration cannot be rolled back. (note: ideally MRs containing schema migrations should not contain feature changes.)
      • Ensure this MR does not add code that depends on these changes that cannot be rolled back.

Documentation/resources

Code review guidelines

Go Style guidelines

Reviewer checklist

• Ensure the commit and MR tittle are still accurate.
• If the change contains a breaking change, verify the breaking change label.
• If the change is considered high risk, verify the label high-risk-change
• Identify if the change can be rolled back safely. (note: all other reasons for not being able to rollback will be sufficiently captured by major version changes).