Add "peer-fingerprint" option for OpenVPN instances

[cmitasch]

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Is your feature request related to a problem? Please describe.

The new OpenVPN instances page is missing the advanced options box found in the old servers page, which is preventing me from manually setting the "<peer-fingerprint></peer-fingerprint>" option. It lists allowed client fingerprints as SHA256-hash, one per line with colons between (see https://github.com/OpenVPN/openvpn/blob/master/doc/man-sections/example-fingerprint.rst)

Describe the solution you like
I want to have an option to set peer-fingerprints in the instances page.