Handle `RegistryAuth::Bearer` for `wkg oci` subcommands

Current implementation does not allow for passing in a bearer token
https://github.com/bytecodealliance/wasm-pkg-tools/blob/dfa2e17128d1b938019d86711f16fb033fddb4f4/crates/wkg/src/oci.rs#L35-L67

And `oras` config files are not checked:
https://github.com/bytecodealliance/wasm-pkg-tools/blob/dfa2e17128d1b938019d86711f16fb033fddb4f4/crates/wkg/src/oci.rs#L41-L41

Start in crates/wkg/src/oci.rs at the referenced lines 35-67 and inspect how the wkg oci subcommands currently construct registry authentication. Verify how RegistryAuth::Bearer should be passed and how oras configuration files are discovered. Done means bearer-token authentication works and relevant oras config files are checked.

Written by the indexing model from the issue text.