examples/docs for sigstore::bundle

[jku]

The sigstore::bundle module seems like the "modern high level sigstore API" out of all modules in the project but it is not mentioned in README and has no examples.

I could try adding an example or two.

[jku]

Might have to rename some things -- the "verify-bundle" example is confusingly named. It's not really a sigstore-rs problem but just a result of sigstore overloading the word bundle.

• I'd like to add examples of verifying and signing the "sigstore bundle" as defined in https://github.com/sigstore/protobuf-specs/blob/main/protos/sigstore_bundle.proto
• Current verify-bundle example verifies a "cosign bundle" (I don't think there is a specification for this one but it's the format cosign produces with sign-blob --bundle=<FILENAME>)