[FLINK-40169][table] Add target option to the EARLY_FIRE hint

[weiqingy]

Part of the FLIP-497 implementation stack under umbrella FLINK-36953. Landing order:

Step	Sub-task	Scope
PR-1a	FLINK-40167	EARLY_FIRE hint surface + option validation (#28353, merged)
PR-1b	FLINK-40168	Thread the hint into the interval join (#28796, approved)
PR-2 (this PR)	FLINK-40169	target option
PR-3	FLINK-40170	Update-producing changelog mode + insert-only guard
PR-4	FLINK-40171	Runtime early-fire emit + retraction
PR-5	FLINK-40172	Processing-time early fire on an event-time join
PR-6	FLINK-40173	State restore coverage
PR-7	FLINK-40174	User-facing documentation

Opened as a draft because it is stacked on #28796, which is approved and awaiting merge. Until that lands, the commit list and diff here also carry PR-1b's commit. Once #28796 merges I will rebase onto master, leaving only this PR's change, and take it out of draft.

What is the purpose of the change

Adds an optional target option to the EARLY_FIRE hint so a hint can be explicitly scoped to a single operator kind. Only interval_join is accepted today, and an omitted target means interval_join, so existing hints keep their meaning. This is a forward-compatibility guard: it keeps a bare EARLY_FIRE hint from silently expanding its scope if other operators honor the hint in the future.

Brief change log

• Add EarlyFireJoinHintOptions.TARGET (optional stringType) and the INTERVAL_JOIN constant.
• The EARLY_FIRE KV option checker validates target against the supported set. Any other value fails planning.
• StreamPhysicalIntervalJoinRule applies the hint only when it targets the interval join, and leaves a hint aimed at another operator kind untouched. That rule-level check is redundant with validation today, since an unsupported value already fails planning. It is there so that a hint aimed at a future operator kind is ignored by this rule rather than misapplied.

Verifying this change

This change added tests and can be verified as follows:

• EarlyFireJoinHintTest: an explicit target='interval_join' still threads earlyFireDelay/earlyFireTimeMode into the exec plan, and an unsupported target value fails planning with a message naming the supported set.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): yes (a new option on the @PublicEvolving EarlyFireJoinHintOptions)
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes (an option on the FLIP-497 hint)
• If yes, how is the feature documented? in the documentation PR at the end of this stack (PR-7)

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CI report:

• 0ef163a Azure: SUCCESS

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[weiqingy]

Hi @RocMarshal, opening PR-2 of the FLIP-497 stack early as a draft so it's queued behind #28796.

Since it's stacked, the commit list and diff here also carry #28796's commit for now. Once that one is merged I'll rebase this onto master, leaving only the target option change, and take it out of draft for review.

Thanks!