KEYS: Introduce load_pgp_public_keyring()

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

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Preload PGP keys from 'pubring.gpg', placed in certs/ of the kernel source
directory.

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>