Stabilise HTTP message signatures support

Now that cURL is adding support for HTTP Message Signatures, I think it is safe to stabilise the implementation we have.

• Wait until httpsig-rs can use stable version of rsa crate. See https://github.com/RustCrypto/RSA/issues/647
• Include http-message-signatures feature in default.
• Similar to cURL, --unstable-m-sig-key should only accept path to a file
• Rename the following options:
  • --unstable-m-sig-id -> --httpsig-keyid
  • --unstable-m-sig-key -> --httpsig-key
  • --unstable-m-sig-alg -> --httpsig-algorithm
  • --unstable-m-sig-comp -> --httpsig-components