Implement JsonSchemaAs for KeyValueMap

[swlynch99]

I'm back with yet another PR! This time we're doing KeyValueMap.

I think this one is probably the most complicated conversion that needs to be done to make the schema work. We need to convert a schema that looks like this

{
  "type": "object",
  "properties": {
    "$key$": "...",
    "a": "...",
  },
  "required": ["$key$", "a"]
}

into this

{
  "type": "object",
  "additionalProperties": {
    "type": "object",
    "required": ["a"],
    "properties": { "a": "..." }
  }
}

On its own that is not to bad but it gets more complicated since we also have to deal subschemas which can be generated from types that make use of #[serde(untagged)] and #[serde(flatten)]. See the doc comment on kvmap_transform_schema for more details.

Notes

• I'm not entirely sure this is correct in the presence of an anyOf subschema that makes use of minProperties and maxProperties (normal ones where they have a $key$ field should work fine). I'm not entirely sure there is one correct behaviour in the case either but to me this seems like a edge case that is pretty far out there so it should be fine.
• It also isn't correct if the $key$ property ends up being defined in some external schema (i.e. $ref points to some random document on the internet). This seems like an edge case and I don't think there's any reasonable behaviour here anyway.

[swlynch99]

The CI errors all seem to be related to the #[warn(unused_qualifications)] lint in Cargo.toml. That doesn't seem related to this PR and when I went to fix them the suggestions from cargo clippy --fix seemed like they would be invalid under #[no_std]. I'll leave this one up to you to fix and/or disable the lints and then rebase once that is done.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.64103% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 66.04%. Comparing base (4c05768) to head (a804257). Report is 1 commits behind head on master.

Files	Patch %	Lines
serde_with/src/schemars_0_8.rs	75.64%	19 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #713 +/- ## ========================================== + Coverage 65.64% 66.04% +0.40% ========================================== Files 38 38 Lines 2343 2421 +78 ========================================== + Hits 1538 1599 +61 - Misses 805 822 +17 ```

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[jonasbb]

Thank you, this looks good. Your comments are good, explain what is going on, and I could follow them. Thanks for getting this to work :)