Initial implementation of Secret Store CryptoKey binding

[jasnell]

This is an initial proof-of-concept implementation of a secret store CryptoKey binding.

// To run the server:
bazel run //src/workerd/server:workerd -- serve samples/secret-store/config.capnp --experimental --verbose --watch

See the samples/secret-store to see how to set it up.

// To test
curl localhost:8080