[FLINK-40592][metrics] Fix PushGateway basic authentication without JAXB

[weicheng-07]

What is the purpose of the change

Fixes FLINK-40592.
Configuring both credentials causes PrometheusPushGatewayReporter to fail during initialization when JAXB is absent. Use JDK Base64 encoding so Basic authentication no longer requires JAXB.

Brief change log

• Add a private connection factory that encodes the Authorization header with UTF-8 and java.util.Base64, preserving the default connection factory and existing credential behavior.
• Add six parameterized cases that isolate JAXB and verify Authorization headers on HTTP PUT and DELETE, including non-ASCII, empty, absent, and incomplete credentials.

Verifying this change

Local verification was performed before applying the patch to the latest master, preserving the two changed files byte for byte.

• ./mvnw -Djdk17 -Pjava17-target -pl flink-metrics/flink-metrics-prometheus clean verify passed on macOS aarch64 with JDK 17 and Maven 3.9.16: 34 tests, including all six new cases; Checkstyle, Spotless, and japicmp passed.
• Before the fix, the same new test reproduces three JAXB initialization errors; the other three cases pass.
• Packaged-JAR checks reproduced the baseline failure. With the fix, all six cases passed using Java 11-targeted artifacts on Java 11 and Java 17-targeted artifacts on Java 17 and 21.
• Community Azure Pipelines build #78940 passed for commit 6f0650c, including compilation, QA checks, all six module test groups, and all four E2E groups.

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.

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CI report:

• 6f0650c Azure: SUCCESS

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