Allow external crates to add their own JsonSchema impls for custom SerializeAs and DeserializeAs types

[swlynch99]

This is written on top of #676.

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Implementing JsonSchema directly on serde_with::Schema<T, TA> works fine when within serde_with. Once you are outside the crate, however, the orphan rules prevent you from adding impls of the form

impl JsonSchema for Schema<T, MyCustomType> { ... }

We would like users to be able to make that impl (or something equivalent) so we need to find a way to make it work. We make this work by adding a new layer of trait indirection.

This PR introduces a new JsonSchemaAs<T> trait, adds a blanket

impl<T, TA> JsonSchema for Schema<T, TA> where TA: JsonSchemaAs<T>

and then changes all the existing impls to be for JsonSchemaAs<T> instead of on Schema<T, TA>.

Now, when a user wants to add json schema support for their own type they can implement JsonSchemaAs<T> for their custom type and the orphan rules will allow it.

As a bonus, this seems to have made the extraordinarily confusing "reached recursion limit" errors that I have been encountering disappear. Which is nice.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (170b6e1) 65.24% compared to head (b0ee3ab) 65.16%.

Files	Patch %	Lines
serde_with/src/schemars_0_8.rs	50.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #679      +/-   ##
==========================================
- Coverage   65.24%   65.16%   -0.09%     
==========================================
  Files          38       38              
  Lines        2256     2268      +12     
==========================================
+ Hits         1472     1478       +6     
- Misses        784      790       +6     

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[swlynch99]

This should be a bit easier to review now that I've rebased onto the latest master :)