Store the list of activated providers in the libctx

Store the list of activated providers in the libctx

The provider config module was storing the list of activated providers
in a global variable. However, because different libctxs can each load
providers via config files we need to keep the list of activated providers
separate and in the libctx.

Partially fixes #15030

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15033)