security: Reduce code duplication between auto_tls_init and the rest of the security package

[itsbilal]

A lot of the primitives in pkg/security/auto_tls_init.go are just shortcuts or
specializations around existing routines in the security package. As part of
this issue, tackle any major instances of code repetition between the two
parts of the package, and have any TLS auto-init/join related code
use existing.

An example of a repetition is CreateCACertAnd Key in auto_tls_init.go
being similar to code in pkg/security/x509.go. Rather, that method tries
to exactly mimic its behaviour. A simple restructure of code in auto_tls_init.go
to exclusively use existing routines elsewhere in the security package
would go a long way in cleaning up repetitive code.

Stems from #60632.

Jira issue: CRDB-7325
Epic: CRDB-6663