Add support for build secrets

[percontation]

docker-compatible --secret id=key,... arg for container build, that works with Dockerfiles with RUN --mount=type=secret

Requires apple/container-builder-shim#69

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Adds support for Dockerfiles that use build secrets (e.g. RUN --mount=type=secret ...)

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[percontation]

I haven't done much Swift before, so watch out for any stupid mistakes I may have made.

One design caveat: I passed secrets by copying the build-args code, because they're functionally the same. But, systems are supposed to take care that the secret values aren't logged or stored anywhere, so HTTP headers are a much riskier way to send them than transporting them separately in a new BuildTransfer. I nevertheless used the headers to send the secrets because 1. it seemed easier 2. I didn't notice anything that would log or record these headers 3. it looked like the headers and BuildTransfers alike are just data sent through gRPC, so, not much practical difference right now.

[JaewonHur]

Hi @percontation, could you resolve conflicts one last time? and we can merge it!