Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Adds two private conversion helpers to translate from the old
signature format to Signature and vice-versa, i.e.
- change "signature" field name to "sig",
- list additional gpg signature field "other_headers" under
  Signature's "unrecognized_fields"

The helpers are used in GPGSigner.sign and (in a follow-up PR)
in GPGKey.verify_signature.

Also removes the no longer needed GPGSignature class.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Assert that GPG Signature instances have the correct extra field,
and test conversion to and from legacy format.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>