Implement Request#signal.

[calavera]

Issue # (if available)

Fixes https://github.com/awslabs/llrt/issues/182

Description of changes

Allow to pass an AbortSignal as an option to a request. Ensure that only AbortSignal types are allowed.

Checklist

• [x] Created unit tests in tests/unit and/or in Rust for my feature if needed
• [x] Ran make fix to format JS and apply Clippy auto fixes
• [x] Made sure my code didn't add any additional warnings: make check
• [ ] Updated documentation if needed (API.md/README.md/Other)

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[calavera]

We also need to check if signal is aborted when we use the request. Similarly the same signal should be passed to response and handle aborted value as well

@richarddavison can you point me to where that's handled? I cannot find the right place to add those checks.

[richarddavison]

We also need to check if signal is aborted when we use the request. Similarly the same signal should be passed to response and handle aborted value as well

@richarddavison can you point me to where that's handled? I cannot find the right place to add those checks.

First place would be to here: https://github.com/awslabs/llrt/blob/a3a9fa73700006c46ebe3171635ffb48a70d440c/src/http/fetch.rs#L175

And then when response is read: https://github.com/awslabs/llrt/blob/a3a9fa73700006c46ebe3171635ffb48a70d440c/src/http/response.rs#L133

I see two solutions:

1. Store a broadcast channel and broadcast on signal so async tasks can be cancelled
2. Periodically check for cancellations between async calls (may load unnecessary data)