router: add filter_state internal redirect predicate

[juanmolle]

Commit Message:
router: add filter_state internal redirect predicate

Adds the envoy.internal_redirect_predicates.filter_state predicate, which decides
whether to follow an internal redirect by comparing a filter state string object (set
earlier in the request by another filter) against a configured value: allow_value
follows only on a match, deny_value follows unless it matches, and allow_if_absent
controls behavior when the object is missing (default: do not follow). No change for
existing configs; the predicate has effect only when added to an InternalRedirectPolicy.

Additional Description:
Uses a raw string with a required oneof allow_value / deny_value rather than a
StringMatcher, because the predicate factory does not receive a CommonFactoryContext
(needed by StringMatcher for regex/custom). This stays additively upgradeable to a
StringMatcher option later without an API break.

Risk Level: Low — self-contained extension, inert unless explicitly configured.

Testing: Unit tests for factory registration and allow/deny/absent behavior. Built
//source/exe:envoy-static and confirmed the extension registers.

Docs Changes: Added the predicate to the internal redirect section of
http_connection_management.rst; API reference generated from the proto comments.

Release Notes: Added changelogs/current/new_features/router__added-filter-state-internal-redirect-predicate.rst.

Platform Specific Features: N/A

[Generative AI usage disclosure: Generative AI (Claude) was used to assist with this change;
I have reviewed and understand all of the code.]

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #45904 was opened by juanmolle.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @adisuissa
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #45904 was ready_for_review by juanmolle.

see: more, trace.