[feature] Support variable references in environment variables

[tonytkachenko]

Is your feature request related to a problem? Please describe.
I'm often frustrated when I need to duplicate or update the same value across many requests because environment variables cannot reference other variables or be composed from parts. This makes switching tokens/IDs or building composed values tedious and error-prone.

Describe the solution you'd like
Allow environment variables to reference other environment variables and support simple interpolation/concatenation.

Examples:
Alias / indirection:

tokenA = some_token_a
tokenB = some_token_b
token = tokenA

Using token in requests should resolve to the value of tokenA (or tokenB if changed).

Composition:

partA = hello
partB = world
whole_thing = ${partA}_${partB}

whole_thing should resolve to hello_world.
Resolution should happen before sending the request, with support for multiple references in one value and clear error handling for circular references.