rust: implement glome verify subcommand

[burgerdev]

This PR adds the verify subcommand to the Rust CLI. Input tags can be truncated outputs of the tag subcommand (i.e., in base64-encoded form), the minimum size of a tag can be adjusted with a flag.

Verify a message tag

Usage: glome verify [OPTIONS] --key <FILE> --peer <FILE> <TAG>

Arguments:
  <TAG>
          Tag to verify

Options:
  -k, --key <FILE>
          Path to secret key

  -p, --peer <FILE>
          Path to peer's public key

  -c, --counter <n>
          Message counter index

      --min-tag-length <n>
          Minimum tag length
          
          Ideally a multiple of 4, defaults to 10 matching the MIN_ENCODED_AUTHCODE_LEN in login/login.h. Must be at least 2 and will be increased to 2 if the argument is lower.
          
          [default: 10]

  -h, --help
          Print help (see a summary with '-h')

Closes #188.

[pkern]

Generally LGTM but one question.

[pkern]

So the minimum tag length would need to be checked by the caller of verify, right? I weakly feel that this is surprising behavior and there should be some minimum tag length setting/flag that you need to pass. I could be convinced otherwise.

[burgerdev]

Makes sense, yes. I added a flag for this and set the default as we do in login.h.

[pkern]

Thanks!