crypto: add sign/verify support for RSASSA-PSS

[tniessen]

Adds support for the PSS padding scheme. Until now, the sign/verify functions used the old EVP_Sign*/EVP_Verify* OpenSSL API, making it impossible to change the padding scheme. Fixed by first computing the message digest and then signing/verifying with a custom EVP_PKEY_CTX, allowing us to specify options such as the padding scheme and the PSS salt length.

Fixes: #1127

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

crypto, src, doc, test