Implement JsonSchemaAs for PickFirst

[swlynch99]

This is the second-to-last type that I've got an implementation queued up for. The only one left after this is Seq.

This one is pretty straightforward: we put the involved schemas in an anyOf subschema. All the schemas except the first are annotated with writeOnly: true to indicate that they are only accepted when deserializing and not emitted when serializing. The one extra detail is that I've used an internal allOf in each element so that setting writeOnly: true works even if the schema of the type is a reference to an external schema.

I have also included tests and a snapshot test to show that everything works.

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 66.49%. Comparing base (5e36083) to head (f4145e3).
Report is 11 commits behind head on master.

Files	Patch %	Lines
serde_with/src/schemars_0_8.rs	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
+ Coverage   66.21%   66.49%   +0.27%     
==========================================
  Files          38       38              
  Lines        2445     2468      +23     
==========================================
+ Hits         1619     1641      +22     
- Misses        826      827       +1     

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[jonasbb]

Thanks, the implementation looks good :) Like all your PRs