req: adding support -addext "subjectAltName=..."

[lealem47]

Adds support for openssl's req -addext flag to specify the subjectAltName (SAN) extension inline on the command line, instead of only via a -config file section.

This now works, mirroring openssl:

./wolfssl req -x509 -newkey rsa:2048 -keyout tls.key -out tls.crt -days 365 -nodes -subj "/CN=192.168.1.2" -addext "subjectAltName=IP:192.168.1.2"

This PR refactors wolfCLU_setAltNames to move some logic into the new wolfCLU_addAltName to avoid duplicate code

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