[sr-dev] [kamailio/kamailio] Add function `secsipid_sign_privkey` to SECSIPID module (Issue #3620)

### Description

Add a function `secsipid_sign_privkey(sheaders, spaypload, keyData)` that accepts a pseudovariable with the PEM data analagous to the difference between the following functions:

- `secsipid_build_identity(origTN, destTN, attest, origID, x5u, keyPath)` - `secsipid_build_identity_prvkey(origTN, destTN, attest, origID, x5u, keyData)`

With the difference being that ``secsipid_build_identity()` takes a key file as it's argument, and ``secsipid_build_identity_privkey()` allows having the key data in a pseudovariable.