Add support for string interpolation

[mr0re1]

Example:

  - id: bahcha
    source: modules/scripts/startup-script
    settings:
      runners:
      - type: data
        destination: /tmp/$(vars.path)/arbuz
        content: |
          #!/bin/bash
          echo "Hello $(vars.project_id) from $(vars.region)"

Refactor escaping logic:

• Don't persist escape backslashes in cty.Value representation;
• Don't "escape-back" during HCL-write;
• Keep all escape logic & info to the YAML-unmarshal/marshall code.

Treat every non-HCL-literal YAML string as an expression

• Split string on plain-string and BP-expression by tokenizing it (see tokenizeBpString)
• If it is one plain-string piece, return cty.StringVal;
• If it is one expression piece, return expression without string interpolation;
• return HCL string interpolation expression combining all parts.