Auto merge of #12700 - jeenalee:jeena-requestAPI, r=jdm

Implement the Request API

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This PR implements the [Request API](https://fetch.spec.whatwg.org/#request-class) for the Fetch API, including its attributes and constructor, and introduces changes in relevant files.

This Request integrates `net_traits::request::Request` and `dom::headers`.

There are few related TODOs and comments:
1. `net_traits::request::Request`'s `headers` field does not point to `dom::request::Request`'s `headers_reflector`.
2. Every Constructor step that involves `Readable Stream` object is not implemented.
3. Every Constructor step that involves `entry settings object` or `environment settings object` is not implemented.
4. `./mach build -d` does not report any error, but prints a few warnings about unused variables related to (1) and (2).
5. Enum `ReferrerPolicy` generated by `RequestBinding` does not match `net_traits::request::Request`'s implementation.
6. `Promise`s in Body webidl are commented out.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11895 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests  because tests for the Request API already exists, but this commit does not implement the interface fully.

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