opamp: add environment vars for implementing mTLS

[jackshirazi]

• ELASTIC_OTEL_OPAMP_CERTIFICATE environment variable: The path of the trusted certificate to use when verifying a server’s TLS credentials, this is needed for mTLS or when the server is using a self-signed certificate. No default value.
• ELASTIC_OTEL_OPAMP_CLIENT_KEY environment variable: Clients private key path to use in mTLS communication in PEM format. No default value.
• ELASTIC_OTEL_OPAMP_CLIENT_CERTIFICATE environment variable: Client certificate/chain trust for clients private key path to use in mTLS communication in PEM format. No default value.