Add packets for caching_sha2_password public key retrieval

[Will-Low]

Situation

We were trying to parse packets associated with public key requests/responses when usingcaching_sha2_password authentication. However, we don't believe that these packet types exist yet.

Target

Allow packet parsing of public key requests and responses for caching_sha2_password authentication.

Proposal

1. Create a new module to house details specific to caching_sha2_password authentication. We have a subsequent addition that is also caching_sha2_password-specific.
2. Introduce a packet to represent a public-key request for caching_sha2_password. Note that while this is similar to the one used for sha256_password, it has a different packet identifier.
3. Introduce a packet to represent a public-key response. This is shared between both caching_sha2_password and sha256_password, so it is excluded from the caching_sha2_password module.