feat(api-header): support custom OpenAPI extensions

[kyungseopk1m]

Summary

@ApiParam and @ApiQuery both accept an extensions bag and flatten each entry into x-* keys on the emitted parameter object (@ApiQuery added in #3949). @ApiHeader does not, even though ApiHeaderOptions extends Omit<ParameterObject, 'in'> uses the same schema as the other two.

This PR mirrors that implementation in @ApiHeader:

• adds an optional extensions field to ApiHeaderOptions,
• merges each entry as an x--prefixed key on the parameter metadata (auto-prefixing keys that don't already start with x-),
• works for both method-level and class-level usage.

No behaviour changes for callers that don't use extensions.

Test plan

Added unit tests in test/decorators/api-header.decorator.spec.ts:

• merges extensions into parameter metadata (method level and class level),
• auto-prefixes extension keys without x-,
• asserts no raw extensions key remains on the emitted parameter,
• asserts metadata is unchanged when extensions is not provided.

npm run test, npm run build and npm run lint pass.