Trust store: Overwrite conflicting ssl options and ensure cacerts set (backport #15116)

[mergify]

Proposed Changes

Addresses #15103

lists:keymerge/3 was used to merge required options into user provided ssl options, which assumes the input lists are sorted. No matter if the intention was to preserve user provided options or overwrite them in case of conflict, without sorting, the outcome was unpredictable. It is also somewhat surprising to those who are used to the behaviour of proplists:get_value (like me) that the ssl app (at least since OTP 26) takes the last value not the first when same key with multiple values are provided.

This commit makes sure the input lists are ordered. Also changes the behaviour of the plugin that all keys used by the plugin overwrite user provided values (fail_if_no_peer_cert, partial_chain, verify, verify_fun)

Also if the user did not provide a cacerts or cacertfile option, an empty cacerts list is added, as ssl config validation requires it in case verify_peer is enabled.

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Further Comments

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This is an automatic backport of pull request #15116 done by [Mergify](https://mergify.com).