Support for TLS Certificates as Strings

[thebeanogamer]

Currently, the Consul provider supports being given certificates for Mutual TLS inline (in a variable) or as a file path, in its cert_pem and key_pem fields. The Nomad provider lacks this functionality, as certificates can only be provided as paths. Matching this functionality would be very useful for environments like Terraform Cloud, where the only way to put a file onto a system would be to include it in version control.

This functionality was implemented in hashicorp/terraform-provider-consul#220, and whilst I plan to look at copying these changes to a PR on this repository, my Go and Terraform Provider Development experience is very limited, so that may not be a time efficient solution. Notably, the structure of unit tests seems to be a bit different on that repository, so this will need to be examined when migrating these changes. If the plan is to implement the same or very similar code to the aforementioned PR, I think it would be appropriate for @lawliet89 to give their approval, as it is ultimately their work.

As I say, I plan to look at making these changes myself, but I am happy to test someone else's implementation if they develop one.