Introduce dynamic provider sources

[matejrisek]

Introduce dynamic provider source and version - meaning we can now specify variables for required provider` sources and versions.

Example of what should be possible with these changes:

terraform {
  required_providers {
    tfcoremock = {
      source  = "${var.tfcoremock_src}"
      version = "${var.tfcoremock_ver}"
    }
  }
}

variable "tfcoremock_src" {
  type = string
  const = true
}

variable "tfcoremock_ver" {
  type = string
  const = true
}

Potential future improvements:

• We might get rid of the "legacy" string only resolution and treat everything as an expression to get rid of the old codepath. The reason we don't do it here is because there's a lot of tests failing that need to be changed for that. Not to cause more noise in this already sizable PR I decided not to include those changes here. Also we reduce blast radius by keeping the old behavior in case unforeseen issues occur in the new codepath.

Target Release

1.16.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.