Implemented OpenSSL providers support

[tomaszma1]

Hi guys,

I have implemented support for OpenSSL providers. It's a new feature available from OpenSSL 3.x.x.

Added CMake compile option “-DPAHO_SSL_PROVIDERS”, which translates to “-DOPENSSL_PROVIDERS=1” C/C++ definition turning on OpenSSL providers support.
“MQTTAsync_connectOptions” and “MQTTClient_connectOptions” structures are extended by “providerName” C-string field with provider name. Empty or invalid “providerName” field indicates no provider used.

In function “SSLSocket_createContext()” provider is loaded, provider self-test is performed, key is obtained and assigned to certificate by calling SSL_CTX_use_PrivateKey() function. If any of these operations fail, function is cancelled with an error code.

Related Paho MQTT C++ PR: eclipse-paho/paho.mqtt.cpp#517

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[tomaszma1]

Hello, it's been 3 weeks already. Any decision to approve/reject this PR? Thanks.

[Pascal-Fremaux]

@tomaszma1 Interesting, but I wonder if you have a leak issue with your pkey.
OSSL_STORE_INFO_get1_PKEY() creates a duplicate and someone, someday, has to clean it with EVP_PKEY_free().
No idea how it is managed by SSL_CTX_use_PrivateKey(), but at least you would have a leak if this call fails (error line 701).