fix: schema class can set Meta.unknown

[dbluhm]

This PR enables the Schema class of a model to determine the default unknown behavior. Without this, if you have a schema where you expect extra values to be present, every time deserialize or serialize is called, you must set unknown=INCLUDE. Now, in the Meta class of the Schema, unknown can be set to some value that will be respected as the default if not overrided by method parameter.

While I was at it, I also touched up some of the typing. I'm not fond of using @overload but I think it's the best way to address typing a method where a bool flag impacts the return type.

[codecov-commenter]

Codecov Report

Merging #1885 (d2ea331) into main (a16347d) will decrease coverage by 0.01%.
The diff coverage is 85.10%.

@@            Coverage Diff             @@
##             main    #1885      +/-   ##
==========================================
- Coverage   93.55%   93.53%   -0.02%     
==========================================
  Files         539      539              
  Lines       34526    34559      +33     
==========================================
+ Hits        32300    32326      +26     
- Misses       2226     2233       +7     

[sonarqubecloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
8.4% Duplication

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information