Add support to pass salt length for RSASSA_PSS

[Taowyoo]

Background

We confirmed that we now could not pass a salt length through the rust-mbedtls APIs. However it was found that the function rsa_rsassa_pss_sign_ext does allow us to supply a salt length (note this is in the mbedtls-sys-auto).

Note

When turning on tls13 feature, need to be careful to following PSA specific limitations
From mbedtls-sys/vendor/docs/architecture/psa-migration/psa-limitations.md :

• signature: mbedtls_rsa_rsassa_pss_sign()
  • message hashed externally
  • encoding hash = MGF1 hash (from context, or argument = message hash)
  • salt length: always using the maximum legal value
• signature: mbedtls_rsa_rsassa_pss_sign_ext()
  • message hashed externally
  • encoding hash = MGF1 hash (from context, or argument = message hash)
  • salt length: specified explicitly

Also may need to call psa_crypto_init before any call to above functions.

Acceptance Criteria

• Update the rust-mbedtls to have a function that allows us to pass a salt_length to the sign function or makes use of the current logic (where salt_length == hash_length)
• Ensure there isn’t a performance degradation