Replumbing: New public API to load or add providers

Adding a provider means creating an internal provier object and adding
it to the store.  This allows the addition of built in providers, be it
in the OpenSSL libraries or in any application.

"Loading" a provider is defined broadly.  A built in provider is already
"loaded" in essence and only needs activating, while a provider in a
dynamically loadable module requires actually loading the module itself.
In this API, "loading" a provider does both.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8287)