Ignore unknown Http3Setting Identifier in Http3SettingsFrame - #360

* 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
[https://github.com/netty/netty/pull/15835](https://github.com/netty/netty/pull/15835)
still allowed direct injection of unsupported settings; this PR
effectively deprecates that direct-settings behaviour completely.

---

* [x] **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.

* [x] **Added missing HTTP/3 error codes**

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

* [x] **Added `SETTINGS_H3_DATAGRAM (0x33)`**

  * Introduced the identifier:

    ```java
    SETTINGS_H3_DATAGRAM(0x33)
    ```

  * Added proper handling in both encoder and decoder paths.

* [x] **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`

---

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* [x] Behaviour from PR #15835 is effectively superseded and corrected.

Fixes #[15908](netty/netty#15908)
Port of netty/netty#15909