feat: add support for unknown fields

[Vilsol]

Implements unknown fields as discussed in #437 behind the unknownFields feature flag.

I had to resort to using undocumented API for writing the proto, as it's apparently unintended to be able to just write out arbitrary bytes to the buffer:

(writer as any)['_push'](
  (val: Uint8Array, buf: Buffer, pos: number) => buf.set(val, pos),
  value.length,
  value
);

The test makes sure that decoding -> encoding -> decoding works.

[stephenh]

Wow, this is great; the changes to main.ts were much smaller/cleaner than I anticipated. Nice!

[stephenh]

🎉 This PR is included in version 1.101.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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