Cross-platform verification of RSA PSS signatures

[trishankatdatadog]

Description of issue or feature request:

There are two related issues:

1. The salt length used when creating RSA PSS signatures.
2. And the salt length used when verifying these signatures.

Current behavior:

1. We use a salt length equal to DIGEST_LENGTH when creating these signatures.
2. Same for verifying them. This is a problem when verifying cross-platform signatures (e.g., from the Golang crypto/rsa packages, which uses a different salt length by default as described below; see Allow configuring the possible salt lengths for RSA PSS signatures hashicorp/vault#16549).

Expected behavior:

1. Although optional, and not necessary for the cross-platform verification of signatures, we should use a salt length equal to MAX_LENGTH for the maximum provable security when creating these signatures.
2. We should use a salt length equal to AUTO so that we can automatically verify these signatures regardless of the input salt length.

[trishankatdatadog]

(Related: #262)