Support OpenSSL v3 store providers

[amananas]

OpenSSL3 introduced the providers mechanic, and obsoleted the engine mechanics. The OpenSSL applications now leverage the store provider mechanics to remove the key type selection: the key type is inferred from the key URI.

This change adds support for the OpenSSL store mechanics when available to load private keys, and makes it the default behavior according to OpenSSL guidelines.

Close #3115

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