fix: fix query context precedence layer for datasource-level per-segment timeout

[jtuglu1]

Description

Currently, per-datasource, per-segment timeout can be not set if there's a hardcoded static default query context (druid.query.default.context.perSegmentTimeout). This ensures the evaluation precedence for the per-segment timeout is as follows (from highest to lowest):

1. Query-specified context values (passed in the query payload)
2. Dynamic configs (broker per segment timeout, broker dynamic config, etc.). Former beats the latter for this particular config.
3. Static defaults specified at process boot (druid.query.default.context.perSegmentTimeout)
4. Default values specified in the application code.

Also adds the perSegmentTimeout context key so it's easy to auto-complete in the UI.

Release note

Fix query context precedence layer for datasource-level per-segment timeout.

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.