load provider keys from handle

[mcr]

This code uses the openssl 3.x OSSL_STORE_* APIs to load key objects.
The older d2i_* and PEM_read_* do not interact at all with providers.
This does not solve generating keys using a provider.

[rhenium]

"handle" seems to be a TPM terminology. Since OpenSSL's man page for OSSL_STORE_open() only says that it takes an URI, "uri" would be a better name here.

This needs changes:

• Please add tests.
• The code formatting should match the existing style.
• OSSL_STORE_INFO_get1_PKEY() appears to handle private keys only, not public keys or parameters-only pkeys.
• The second parameter pwd is unused.
• OSSL_STORE and OSSL_STORE_INFO are not freed.

[rhenium]

The older d2i_* and PEM_read_* do not interact at all with providers.

ruby/openssl uses OSSL_DECODER with OpenSSL 3 instead of d2i_*() and PEM_read_*() functions. It does actually interact with providers, as long as the provider supports encoding the key reference into DER/PEM.

Support for OSSL_STORE_open() would be useful. This should resolve #722.