X509_LOOKUP_store: new X509_LOOKUP_METHOD that works by OSSL_STORE URI

X509_LOOKUP_store: new X509_LOOKUP_METHOD that works by OSSL_STORE URI

This is a wrapper around OSSL_STORE.

This also adds necessary support functions:

- X509_STORE_load_file
- X509_STORE_load_path
- X509_STORE_load_store
- SSL_add_store_cert_subjects_to_stack
- SSL_CTX_set_default_verify_store
- SSL_CTX_load_verify_file
- SSL_CTX_load_verify_dir
- SSL_CTX_load_verify_store

and deprecates X509_STORE_load_locations and SSL_CTX_load_verify_locations,
as they aren't extensible.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8442)