Support CoSERV request-response with signed output

[paulhowardarm]

Summary

The PR #29 introduced the first stage of support for the CoSERV request-response API, but it is limited to unsigned output from the server. This request is to enhance the API to support signed output as well.

Detail

• The coserv::QueryRunner type currently offers the execute_query_unsigned() method. The request is basically to introduce a new method alongside that called execute_query_signed(), which essentially does the same job, but sets the media type as application/coserv+cose instead of application/coserv+cbor. This will trigger the server to produce signed rather than unsigned output.
• There is a dependency on signed CoSERV support being added to the coserv-rs crate.
• There's a design question to decide who verifies the signature - should verification be bundled into the client library, or do we make that the caller's problem?