Support joining lists when lists with same name are imported into an environment.

[MitchellGerdisch]

Hello!

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Issue details

Take the following example where constructs with the same names are declared in two different environments:

EnvironmentA:

values:
  MyList:
    - alpha
    - beta
  MyDictionary:
    item1: "thing1"
    item2: "thing2"

Environment B:

values:
  MyList:
    - delta
    - gamma
  MyDictionary:
    item3: "thing3"
    item4: "thing4"

When these environments are imported in a third environment, it would be useful (if not expected) that the two lists would be concatenated like the dictionary is.

EnvironmentC:

imports:
  - escproj/EnvironmentA
  - escproj/EnvironmentB

Currently, this produces:
(Note the list just contains the elements from the list in EnvironmentB.)

{
  "MyDictionary": {
    "item1": "thing1",
    "item2": "thing2",
    "item3": "thing3",
    "item4": "thing4"
  },
  "MyList": [
    "delta",
    "gamma"
  ]
}

But it would be helpful to have

{
  "MyDictionary": {
    "item1": "thing1",
    "item2": "thing2",
    "item3": "thing3",
    "item4": "thing4"
  },
  "MyList": [
    "alpha",
    "beta",
    "delta",
    "gamma"
  ]
}

### Affected area/feature

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[nyobe]

This is an unfortunate limitation of the JSON Merge Patch behavior that we use for merging environments: https://www.rfc-editor.org/rfc/rfc7386#:~:text=There%20are%20a,in%20an%20array.

But if we were to add a fn::concat function then this could be solvable by explicitly concatenating the values from the other two environments:

# EnvironmentC
imports:
  - escproj/EnvironmentA
  - escproj/EnvironmentB
values:
  MyList:
    fn::concat:
      - ${imports["escproj/EnvironmentA"].MyList}
      - ${imports["escproj/EnvironmentB"].MyList}

---

edit: we now have fn::concat!