Update Spec - Env. Variable Interpolation in JSON Inputs

With a recent PR #766, we are adding support to interpolate input values using env. variables such that:

```
os.SetEnv("USERNAME", "MY_USERNAME")
os.SetEnv("PASSWORD", "MY_PASSWORD")
```

manifest.yaml:

```
inputs:
type: json
GitHub_credentials: {
username: ${USERNAME},
password: ${PASSWORD}
}
```

Action is composed with `username` set to `MY_USERNAME ` and `password` set to `MY_PASSWORD`.

With manifest.yaml:

```
inputs:
type: json
GitHub_credentials: {
username: $${USERNAME},
password: $${PASSWORD}
}
```

Action is composed with `username` set to `${USERNAME} ` and `password` set to `${PASSWORD}` escaping interpolation with double dollar sign, following terraform syntax at https://www.terraform.io/docs/configuration/interpolation.html.