Add option to apply security rule only to certain signatures.

[ni4]

Description

Currently hash security rules apply to all signatures. However, it may be desirable to distinguish between key signatures and data signatures, so we should add corresponding flag to the rnp_add_security_rule().
Probably it could also worth to allow SHA1 for key's signatures by default.

[antonsviridenko]

Why do we need that?

[ni4]

Please see the discussion here: https://mailarchive.ietf.org/arch/msg/openpgp/KeTgr5qdLlu3eKQSHnG-3qCmx4Q/
Basically, while it is simple to avoid to sign data using the SHA1 hash algorithm, it could become harmful for the keys.