Support certificate and key in PEM string format

[colincmac]

Conlfuent.kafka and librdkafka supports authentication certificate in pem string format using "ssl.certificate.pem" and "ssl.key.pem" attributes. You do not have to maintain certificate as file within application or certificate store with this support. This extension does not supports these methods of specifying certificate.

Adding this capability would allow referencing the PEM string as a KeyVault secret, since KeyVault only supports certificates in PFX formats.

There's a similar issue logged here:
Azure/azure-functions-kafka-extension#310