configurable hash functions

[jschicktanz]

What this PR does / why we need it:

• makes hash function configurable
• pass in hash function in rsa.Sign() and rsa.Verify() calls instead of passing in 0. this will apply EMSA-PKCS1-v1_5 encoding (https://datatracker.ietf.org/doc/html/rfc3447#section-9.2) before signing the data, which is part of standard RSASSA-PKCS1-v1_5 (https://datatracker.ietf.org/doc/html/rfc3447#section-8.2). (!) Attention: this change will break existing signatures

Which issue(s) this PR fixes: Fixes #

Special notes for your reviewer:

Release note:

- pass in hash function in rsa.Sign() and rsa.Verify() calls instead of passing in 0. this will apply EMSA-PKCS1-v1_5 encoding (https://datatracker.ietf.org/doc/html/rfc3447#section-9.2) before signing the data, which is part of  standard RSASSA-PKCS1-v1_5 (https://datatracker.ietf.org/doc/html/rfc3447#section-8.2). (!) Attention: this change will break existing signatures.

[gardener-robot]

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