[Target 390]Serialize BufferAsset as CCON

[shrinktofit]

Re: #

Changelog

• Adjust BufferAsset so that it can be serialized as CCON format.

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Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

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Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.

[dumganhar]

Why remove the public API validate?

[dumganhar]

The overridden get/set _nativeAsset is removed. Will it break the compatibility?
The old get _nativeAsset return this._buffer, but now it returns this._file from its parent class Asset.

[shrinktofit]

As the method documented, this is an engine private interface and has been deprecated since v3.5.0 and is explicitly starts with an underscore. We can directly removed it in v3.9.0.