[RORDEV-1927] Xpack authentication header creator class refactoring

[coutoPL]

This PR simplifies and documents the authentication header serialization logic across all Elasticsearch versions
(6.7.x - 9.2.x):

Changes:

• Removed version conditionals: Eliminated version-specific checks that were always true (all supported ES
  versions are ≥ 6.7.0/7.0.0/8.2.0)
• Added inline documentation: Added clear comments explaining each serialization field (internal user flag, realm
  info, authentication type, metadata)
• Fixed code style: Corrected whitespace in if (isInternal) conditional
• Cleaned up imports: Removed unused TransportVersions import in ES 8.11+ versions
• Improved consistency: Unified serialization format across all 33 ES version modules

The authentication header serialization now unconditionally writes all required fields in the expected order,
making the code more maintainable and easier to understand.

Summary by CodeRabbit

• Bug Fixes
  • Consistent authentication header encoding across transport versions to improve reliability.
  • Fixed realm metadata emission so realm and lookup flags are always present and correctly encoded.
  • Clarified internal vs. regular user serialization so internal accounts use a minimal payload while regular users include full profile fields.