Zig 0.15.1 and POST Policy Support

[BTOdell]

Core Changes

• Updated code to Zig 0.15.1
• Added POST Policy support including pre-signing feature: https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-HTTPPOSTConstructPolicy.html
• Basic tests
• Manual test against MinIO with front-end HTML form

Additional Changes

Removed archive directory

I'm not sure why there is an 80MB TAR archive in the source tree. If you're trying to enable support for build.zig.zon, you don't need to point it to a tar archive in the source tree. You can just reference the entire repo by tag or by hash:

.s3 = .{
    .url = "git+https://github.com/C4-Loop/zig-s3.git?ref=main#577d3fb902c6c4d0a5cddfa9f4868a76051aebc8",
    .hash = "zig_s3-0.2.0-lxQihLHpAQDFGpmLrjYDvVaAFAyyIqthPGhwd6m7J7eJ",
},

docker-compose.yaml

I added a docker-compose.yaml file to bring up a local instance of MinIO on port 9000.
All of the tests and CI automation should use a local container instead of putting cloud credentials in a .env file. In fact, the .env file shouldn't be included in the source tree at all.