Added sts credentials generation per run

[rverma-dev]

This pr will enable to use IRSA or any role based credentials with terraform cloud out of box. The way its implemented is it will paas the temporary credentials of the workspace sync runner to the cloud instance before every run.

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2 out of 4 committers have signed the CLA.

• kunalvalia
• rverma-dev
• Abhishek Verma
• mvkrishna86

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[tsunamishaun]

@rverma-nsl I was going to create an issue for being able to assume a role prior to running which is a feature I'm implementing with Terragrunt/Atlantis and would love to see as part of this operator. The assume role could be a part of the workspace spec and implemented very similar to how you've implemented the credential work here. Thoughts?
cc'ing @redeux @koikonom for feedback as well?

edit: Realizing now that the tfc agent is what would be assuming a role per run but I believe that is closed source. I guess the only way forward is to build "assume_role" into the provider block for all our Terraform?