Add JsonSchema impls for BytesOrString and BoolFromInt

[swlynch99]

This PR continues where #679 left off. Since the impls get more complicated as we go on I will be chunking them up into smaller PRs so that reviewing them stays manageable. I also don't plan on doing more than 1 or maybe 2 PRs concurrently. I just considered #682 important enough that it should jump the queue.

Details

• JsonSchemaFor impls for both BytesOrString and BoolFromInt
• Tests for both of the above

Notes

• BytesOrString's schema is represented as an enum that can be either a string or an array of bytes. The one difference is that the string representation is marked as "writeOnly" as it is never emitted.
• I originally also did something similar for BoolFromInt<Flexible> but I didn't think that this was useful compared to the extra confusion when reading the schema. I have changed it so it's schema is just that of an integer.

[codecov]

Codecov Report

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

Comparison is base (88e9879) 65.16% compared to head (cc9a723) 65.19%.

Files	Patch %	Lines
serde_with/src/schemars_0_8.rs	68.42%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #683      +/-   ##
==========================================
+ Coverage   65.16%   65.19%   +0.02%     
==========================================
  Files          38       38              
  Lines        2268     2287      +19     
==========================================
+ Hits         1478     1491      +13     
- Misses        790      796       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.