Implement a way to access an input value of an adhoc aciton while evaluating its output

Currently, when we define an adhoc action we can define it's output as a YAQL/Jinja expression like this:

my_action:
  base: std.http
  base-input:
    url: ...
    headers ...
  input:
    - param1
    - param2
  output: <% [an expression where "$" refers to the output of the base action] %>

However, while evaluating the output it's impossible to access input values of the adhoc actions, for example, "param1" in the example above.

One way to solve this problem is to provide a function to access input values. For example:

my_action:
  base: std.http
  base-input:
    url: ...
    headers ...
  input:
    - param1
    - param2
  output: <% input().param1 %>

This example assumes that the input() returns an input context of the adhoc action.

NOTE: when we evaluate expressions in Mistral, seems like we always provide same YAQL/Jinja functions, no matter what context we use, e.g. evaluating "publish" or calling an adhoc aciton. We need to look at how to load different function sets depending on a situation.