feat(unstable): Deno.S3Client and Deno.s3

[bartlomieju]

Adds a built-in client for S3-compatible object storage (AWS S3, MinIO,
Cloudflare R2, Backblaze B2, etc.) behind --unstable-s3, so objects can
be read, written, listed and presigned without pulling in a third-party
SDK:

const s3 = new Deno.S3Client({ bucket: "my-bucket" });
await s3.write("hello.txt", "hello world");
const text = await s3.file("hello.txt").text();
const url = await s3.presign("hello.txt", { expiresIn: 3600 });

It is implemented as a JS-only lazy loaded extension (ext/s3, no new ops)
on top of fetch and WebCrypto: requests are signed with AWS Signature
Version 4 (header auth for requests, query auth for presigned URLs), and
streaming writes go through multipart uploads. Configuration falls back to
the usual S3_*/AWS_* environment variables, and a default
env-configured client is exposed as Deno.s3. Network access requires
--allow-net; environment fallbacks only require --allow-env when
explicit options are not provided.

Verified against a mock S3 server that re-computes both signature flavors
server-side, covering round-trips (text/JSON/binary/Blob/stream), keys
needing URI escaping, ranged reads via slice(), a 12 MiB multipart
upload, ListObjectsV2 paging, presigned GET/PUT consumed by plain fetch,
and s3://bucket/key paths.

[iuioiua]

This feels like the runtime doing too much, which I think Bun gets wrong 😕

[Hajime-san]

• [Feature] Built-in S3 support #27840 (comment)

Please do not rely on third party closed source services.

AWS S3 may appear to be a stable interface, but it is being updated day by day.
https://docs.aws.amazon.com/AmazonS3/latest/userguide/WhatsNew.html

There have been breaking changes in the past.

• r/aws_s3_bucket_[acl|logging]: remove display_name attributes hashicorp/terraform-provider-aws#44091

Bun's S3 API is already abandoned due to its difficulties of testing.

• https://github.com/oven-sh/bun/issues?q=is%3Aissue%20state%3Aopen%20label%3As3

I predict that this API will become unusable in three years while it may seem convenient now.