PGPLIB: Basic packet parser

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

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Provide a simple parser that extracts the packets from a PGP packet blob
and passes the desirous ones to the given processor function:

	struct pgp_parse_context {
		u64 types_of_interest;
		int (*process_packet)(struct pgp_parse_context *context,
				      enum pgp_packet_tag type,
				      u8 headerlen,
				      const u8 *data,
				      size_t datalen);
	};

	int pgp_parse_packets(const u8 *data, size_t datalen,
			      struct pgp_parse_context *ctx);

This is configured on with CONFIG_PGP_LIBRARY.

Changelog

v0:
- fix style issues (Roberto Sassu)
- declare pgp_to_public_key_algo (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>