feature: BEAMMP_PROVIDER_DISABLE_MP_SET environment variable (#461)

This pull request adds BEAMMP_PROVIDER_DISABLE_MP_SET variable
(true/false/1/0) to disable MP::Set for providers.

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include/Env.h

@@ -26,6 +26,7 @@ enum class Key {
     // provider settings
     PROVIDER_UPDATE_MESSAGE,
     PROVIDER_DISABLE_CONFIG,
+    PROVIDER_DISABLE_MP_SET,
     PROVIDER_PORT_ENV,
     PROVIDER_IP_ENV
 };