Allow SASL_EXTERNAL to be a multiline string or a path

[arthsmn]

This would allow you to have your certificate in the config instead of a path.

[osa1]

cc resident SASL external expert @trevarj.

[arthsmn]

Also, it would be nice to allow home as a tilde and environment variables in the path.

[osa1]

Also, it would be nice to allow home as a tilde and environment variables in the path.

This was previously requested in #192. I would be happy to review and merge if someone implements it.

[trevarj]

Seems reasonable. We can change the pem field to be a String here and then parse accordingly.