Support optional serialization/deserialization of map with non-string key type

[pjfanning]

Is your feature request related to a problem? Please describe.

Based on FasterXML/jackson-module-scala#316 and some other issues that have popped up where users want to work with Map<KeyClass, ValueClass> where the KeyClass can't be readily represented as a String.

Describe the solution you'd like

Typically, Maps (eg Map(1 -> "a", 2-> "b")) are serialized as something like:

{
  "1": "a",
  "2": "b"
}

But we could have a way to annotate this Map so that it gets serialized as:

[
  {
    "key": 1,
    "value": "a"
  },
  {
    "key": 2,
    "value": "b"
  }
]

The names of the JSON elements key and value could be made configurable.

This latter serialization could allow the key value to be represented as a JSON object or array.

This serialization might be useful for someone who wanted to JSON serialize a Map as opposed to using Java serialization.

Additional context
Not urgent. Just opened as a discussion topic. Users can currently write their own custom serializers and deserializers to achieve this.

[cowtowncoder]

Yeah this is sort of at the edge of what Jackson does do (allow support for slightly different structure/formatting) and does not (general structural transformations). I think I have heard this being desired occasionally so I think there is some demand for supporting this.

From API perspective, @JsonFormat(shape = xxx) could work, although might have to use funny semantics (default format would be NATURAL, but OBJECT seems bit ambiguous). And then again ability to specify overrides for "key" and "value" would require something different.

I'll leave this here for collecting more ideas (and finding someone with time and itch, too). Perhaps it could be tackled as some sort of datatype module, wherein more transformations could be applied...