feat: add DeadlinePriority plugin in intra-flow dispatch policy

[googs1025]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Introduce a new intra-flow dispatch policy named "DeadlinePriority" that
schedules requests based on deadline urgency, computed as EnqueueTime +
EffectiveTTL. Requests without a valid TTL are treated as lowest priority
and ordered by FCFS for fairness.

Which issue(s) this PR fixes:

Fixes #1913

Does this PR introduce a user-facing change?:

add DeadlinePriority plugin in intra-flow dispatch policy

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[googs1025]

@LukeAVanDrie
sorry to bother.
This is currently a draft, but could you please take a look and check if the implementation is reasonable? I will then proceed with the next step of writing test cases.

[LukeAVanDrie]

nit: likely copy-paste error

[LukeAVanDrie]

Initial pass LGTM, but I will take a deeper look first thing Monday morning. Thanks for contributing!

I want to double check how I default EffectiveTTL if undefined.

Regarding QueueCapability pairings, this will be validated automatically by the Flow Registry. We guarantee the paired queue / policy are compatible, so no need to document this in your go-docs. The RequiredCapabilities function is sufficient.

As you may have noticed, pretty much every IntraFlow impl will just return PeekHead. When I align this with the actual EPP plugin model, I will probably make the official plugin the comparator, not the policy impl (which is just imperative code boilerplate). I can handle migrating your policy then.

There should be a README.md which gives a (slightly outdated) policy overview. It also described how to get auto-test coverage for the black box contracts that the orchestrating engine relies upon. Basically, you'll get some free test coverage simply by registering your policy.

[LukeAVanDrie]

Nit: An alternative policy name here may be Earliest Deadline First (EDF). This is the conventional name and a bit more expressive. If GAIE ever supports TTFT latency targets we can use the strictest deadline (TTL vs target, if defined) as the deadline.