Ignore unknown Http3Setting Identifier in Http3SettingsFrame

[normanmaurer]

• According to the spec https://www.rfc-editor.org/rfc/rfc9114.html#section-7.2.4-9 , endpoints must ignore unknown settings identifiers. Netty’s previous behavior incorrectly stored unknown settings values, which contradicts RFC behaviour.
• A HTTP/3 error code was missing - H3_DATAGRAM_ERROR
• The SETTINGS_H3_DATAGRAM (0x33) identifier was not added.
• Prior behaviour fixed in Backward Compatible - Http3SettingsFrame abstraction netty#15835 still allowed direct injection of unsupported settings; this PR effectively deprecates that direct-settings behaviour completely.

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• Ignore unknown HTTP/3 SETTINGS identifiers

  • Updated parsing logic to skip unknown settings identifiers:

    java // When Non-Standard/Unknown settings identifier present - Ignore if (Http3SettingIdentifier.fromId(key) == null) { return value; }

  • Unknown settings are no longer stored in Http3Settings.

  • Fully aligns with HTTP/3 specification behaviour.

• Effectively deprecates the old direct-settings behavior introduced/fixed in PR #15835.

• Added missing HTTP/3 error codes

  • Implemented the remaining H3 error codes defined in the RFC.
  • Ensures accurate error propagation and complete protocol support.

• Added SETTINGS_H3_DATAGRAM (0x33)

  • Introduced the identifier:

    java SETTINGS_H3_DATAGRAM(0x33)

  • Added proper handling in both encoder and decoder paths.

• Added unit tests

  • Tests covering:

    • Ignoring unknown setting identifiers
    • Correct handling of SETTINGS_H3_DATAGRAM
    • Verification of newly added error codes
    • Ensuring unknown settings do not appear in Http3Settings

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• Netty now correctly ignores unknown HTTP/3 settings, as required by the specification.
• Http3Settings no longer stores invalid or non-standard identifiers.
• Full support for SETTINGS_H3_DATAGRAM (0x33).
• Complete coverage of HTTP/3 error codes.
• Added unit tests ensure long-term correctness.
• Behaviour from PR #15835 is effectively superseded and corrected.

Fixes #15908 Port of netty/netty#15909