[BP-2.3][FLINK-40592][metrics] Fix PushGateway basic authentication without JAXB - #29170

What is the purpose of the change

Backport of #29151 (master commit 189b6ca8313ada196596cb53315866e7a531d441) to release-2.3 for FLINK-40592.

Fix PrometheusPushGatewayReporter initialization with Basic authentication when JAXB is absent.

Brief change log

• Encode the Authorization header with JDK Base64 and UTF-8, preserving the default connection factory and existing credential handling.
• Include six JAXB-isolated regression cases covering Authorization headers on PUT and DELETE requests.

Verifying this change

• ./mvnw -Djdk17 -Pjava17-target -pl flink-metrics/flink-metrics-prometheus clean verify passed on release-2.3 for commit 10eb8b30 with Maven 3.8.6 and JDK 17: 34 tests, including all six authentication cases; Checkstyle, Spotless, and japicmp passed.
• The original 28 module tests pass on the unmodified branch. With the regression test added, the unfixed reporter produces three expected JAXB initialization errors; the other three cases pass.
• Community CI is pending.

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 — the reporter is @PublicEvolving; only private implementation changes, with no public signature or configuration changes.
• The serializers: no.
• The runtime per-record code paths (performance sensitive): no.
• Anything that affects deployment or recovery: no; the change is limited to metrics reporter authentication.
• The S3 file system connector: no.

Documentation

• Does this pull request introduce a new feature? no.
• If yes, how is the feature documented? not applicable.

Was generative AI tooling used to co-author this PR?

• Yes (OpenAI Codex)

Generated-by: OpenAI Codex 0.153.4