Add support for encrypted TLS private keys

[chrisdoc]

Context:
We use TLS Certificates with an encrypted private key to connect to our Kafka brokers. Therefore I thought it would be a good idea to add support for encrypted keys to benthos.

Solution:
Decode the provided key and check if it is encrypted and try to decrypt it with the user provided password. I have added test for both certificates as file and string.

[Jeffail]

Thanks @chrisdoc looks great, just need to add an additional entry to https://github.com/benthosdev/benthos/blob/main/internal/tls/docs.go#L44 in order for the documentation to match.

[chrisdoc]

@Jeffail thank you for the fast feedback; I went ahead and added the missing documentation update :)

[Jeffail]

Awesome, thanks @chrisdoc, extra ❤️ for adding more tests