Implement JsonSchemaAs for OneOrMany

[swlynch99]

Here's the next type. This one implements JsonSchemaAs for OneOrMany. It is fortunately much simpler than KeyValueMap.

For OneOrMany the transformation we want to do is basically to take this schema

{ "$ref": "..." }

and transform it into

{
  "anyOf": [
    { "$ref": "..." },
    {
      "type": "array",
      "items": { "$ref": "..." }
    }
  ]
}

that is, we change it so it can take either an array or a single value. The only difference between the PreferOne and the PreferMany variants is that the inner object is marked as writeOnly: true.

I have also included some tests that should cover pretty much everything.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 66.21%. Comparing base (bb2ab60) to head (5e36083).
Report is 5 commits behind head on master.

Files	Patch %	Lines
serde_with/src/schemars_0_8.rs	83.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
+ Coverage   65.64%   66.21%   +0.57%     
==========================================
  Files          38       38              
  Lines        2343     2445     +102     
==========================================
+ Hits         1538     1619      +81     
- Misses        805      826      +21     

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[jonasbb]

That looks great :) Thank you.

[jonasbb]

I published your new JsonSchemaAs implementations and fix for the serde_as macro in v3.7.0