dvc.yaml: introduce set keyword

[skshetry]

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On top of #4734
Here's a wiki documenting the feature: https://github.com/iterative/dvc/wiki/Parametrization

This PR introduces set keyword, which can be used to alias a variable.
Some examples:

1. Setting variable inside for a stage to be DRY

stages:
  build:
    set:
      item: "value"
    cmd: python script.py --thresh ${item}
    always_changed: ${item}

2. set can set list to a variable. But, it is not allowed to contain nested list or nested dictionary inside of it.

stages:
  build:
    set:
      item:
      - foo
      - bar
      - baz
      thresh: 10
    cmd: python script.py --thresh ${thresh}
    # note: DVC does not allow to change schema like this right now
    # just an example of how you could potentially use this
    outs: ${item}

3. set can set dict to a variable. But, it is not allowed to contain nested list or nested dictionary inside of it.

stages:
  build:
    set:
      item:
        foo: foo
        bar: bar
      thresh: 10
    cmd: python script.py --thresh ${thresh}
    outs: ${item}

4. set to create an alias

use: params.json
stages:
  build:
    set:  # optional, loop-wide effect
      data: ${models}
    foreach: ${data}
    in:
      set:  # optional, affects only one instance/iteration
        thresh: ${item.thresh}
      cmd: command --value ${thresh}

NOTE: set is always evaluated first, before foreach, so it's available for iteration on foreach. Similarly, cmd and friends are also evaluated later after set and foreach( if it exists).

Also, note that joining strings is not supported with "set":
eg:

set: 
  # not supported
   url: ${base_url}/${filename}
   # also, not supported
   name: My name is ${name}