[RESTClient] Add placeholder for input parameters

[YaSuenag]

Types of changes

• Enhancement (project structure, spelling, grammar, formatting)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

A description of the changes proposed in the Pull Request

We've introduced to apply environment variables into the manifest. This PR addes feature to apply input parameters into manifests.

Due to this change, we have to specify kinds of values into top of placeholder like ${env:xxx} or ${inputs:xxx}. I updated document about this.