tls: add `system` and `combined` CA pool modules (#7406)

* feat: add system and combined CA pool modules

* fix: combining pools using `CertificateProvider`

* fix: lint issue

* chore: caddyfiletests
* doing it for first time, so not sure if its right.

* fix: use `x509` native addCert

* chore: explicit err handling

* Apply suggestion from @mohammed90

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Co-authored-by: Mohammed Al Sahaf <mohammed@caffeinatedwonders.com>