Include signature type in GpgSig, to support SSH, x509, etc.

[Warbo]

The existing commit parser/printer were hard-coded for PGP signatures, and stripped off their BEGIN/END lines. That threw an error for commits signed with other schemes like SSH or x509 ("expected first line of sig to be a single space or version"), and it loses information about what type of signature was present; which is bad for those inspecting the IPLD DAG, and it prevents the codec from correctly encoding commit objects.

This change generalises the signature parsing, to accept any -----BEGIN and -----END markers. It also includes those markers in the GpgSig value, so signed commits can roundtrip the codec, and the DAG will show what the signature type is.

Note that this changes the contents of the signature field exposed to IPLD! Here's what a PGP-signed commit used to look like:

$ ipfs dag get baf4bcfastgjlbc4cb4pwvazfmxrs6duxojydbsq | jq .
{
  "author": {
    "date": "1772632573",
    "email": "chriswarbo@gmail.com",
    "name": "Chris Warburton",
    "timezone": "+0000"
  },
  "committer": {
    "date": "1772632573",
    "email": "chriswarbo@gmail.com",
    "name": "Chris Warburton",
    "timezone": "+0000"
  },
  "encoding": null,
  "mergetag": [],
  "message": "Signed commit for testing\n",
  "other": [],
  "parents": [
    {
      "/": "baf4bcfafx5b5ehckbgrxs3w5vw4p3l6jnwchxri"
    }
  ],
  "signature": " \n iQEzBAABCgAdFiEEL4FPY3Lp/jQ0Jiq+seTexiNfLroFAmmoOf0ACgkQseTexiNf\n Lrp3QAf9Ee+oNHjHT9aKCRkUyaRn5ZUV9OxmxskD5hDOz7iLnZ84DIx4WNl7QT+x\n kw15VPyrryr16ES/J4HBb9aaMLDb9ns98vmrPUJ6YPKYWoD+0750GM/+RBe9HLsJ\n woTaADmn9ocSym53b32t6tTLvt2ow8J2UbfjijLqsEkqTH+E+T0qzxwlscZ6xWT8\n z40y88lbvIjuyxV8SirnoJtNra91QPuWKmkJCDE7OM4WGA/tU62Za5z4J8q4WZ9A\n +9K0poBMrEpsBRazx8XKNY1w/4v66vKEvY+q8kcH0zGKbX6uYnxsG/kr6bfLIQMU\n 7KdD0xv+QsGw5uhjhD7WLK8ziJ/x0w==\n =vyj/\n",
  "tree": {
    "/": "baf4bcfcthputu4uf5w5m66gfj4szgbazp5agqjy"
  }
}

With this change, the signature field will become:

  "signature": " -----BEGIN PGP SIGNATURE-----\n iQEzBAABCgAdFiEEL4FPY3Lp/jQ0Jiq+seTexiNfLroFAmmoOf0ACgkQseTexiNf\n Lrp3QAf9Ee+oNHjHT9aKCRkUyaRn5ZUV9OxmxskD5hDOz7iLnZ84DIx4WNl7QT+x\n kw15VPyrryr16ES/J4HBb9aaMLDb9ns98vmrPUJ6YPKYWoD+0750GM/+RBe9HLsJ\n woTaADmn9ocSym53b32t6tTLvt2ow8J2UbfjijLqsEkqTH+E+T0qzxwlscZ6xWT8\n z40y88lbvIjuyxV8SirnoJtNra91QPuWKmkJCDE7OM4WGA/tU62Za5z4J8q4WZ9A\n +9K0poBMrEpsBRazx8XKNY1w/4v66vKEvY+q8kcH0zGKbX6uYnxsG/kr6bfLIQMU\n 7KdD0xv+QsGw5uhjhD7WLK8ziJ/x0w==\n =vyj/\n -----END PGP SIGNATURE-----",