KEYS: PGP data parser

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

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Implement a PGP data parser for the crypto key type to use when
instantiating a key.

This parser attempts to parse the instantiation data as a PGP packet
sequence (RFC 4880) and if it parses okay, attempts to extract a public-key
algorithm key or subkey from it.

If it finds such a key, it will set up a public_key subtype payload with
appropriate handler routines (DSA or RSA) and attach it to the key.

Thanks to Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> for pointing
out some errors.

Changelog

v0:
- remove declaration of pgp_to_public_key_algo (Roberto Sassu)
- replace prep->type_data with prep->payload.data (Roberto Sassu)
- replace public_key_destroy() with public_key_free() (Roberto Sassu)
- use asymmetric_key_generate_id() to generate the key identifiers
  (Roberto Sassu)
- add raw_fingerprint to pgp_key_data_parse_context structure
  (Roberto Sassu)
- replace algorithm identifiers with strings (Roberto Sassu)
- don't parse MPIs (Roberto Sassu)
- don't process Public-Subkey packets (only one key will be created)
  (Roberto Sassu)

Signed-off-by: David Howells <dhowells@redhat.com>
Co-developed-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Tianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>