[Refactor] Move OpenSSL providers from global to libs_ctx

[Refactor] Move OpenSSL providers from global to libs_ctx

Previously, OpenSSL 3.0+ providers (legacy and default) were stored in
static global variables. This is not a good architecture as these resources
should be managed alongside other library contexts.

This commit refactors the code to store SSL providers in the
rspamd_external_libs_ctx structure:
- Add ssl_legacy_provider and ssl_default_provider fields to libs_ctx
- Pass libs_ctx to rspamd_openssl_maybe_init() to store providers there
- Remove rspamd_openssl_cleanup() function - cleanup now happens in
  rspamd_deinit_libs() when the libs_ctx is freed
- Remove global variables and manual cleanup calls

This provides better resource management and clearer ownership of
OpenSSL provider lifecycle.