Replumbing: give the possibility for the provider to create a context

OSSL_provider_init() gets another output parameter, holding a pointer
to a provider side context.  It's entirely up to the provider to
define the context and what it's being used for.  This pointer is
passed back to other provider functions, typically the provider global
get_params and set_params functions, and also the diverse algorithm
context creators, and of course, the teardown function.

With this, a provider can be instantiated more than once, or be
re-loaded as the case may be, while maintaining instance state.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8848)