custom date coding via closures

[swhitty]

Adds custom closure based strategy to the recently added Date Encoding/Decoding strategy allowing any date format to be used.

Updates .iso8601() strategy to pass a closure that includes the ISO8601Formatter.

Catches errors from these closures and wraps in DecodingError / EncodingError.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.07692% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.80%. Comparing base (3d6ca2b) to head (9c5fc8e).

Files with missing lines	Patch %	Lines
Sources/KeyValueDecoder.swift	95.65%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   99.89%   99.80%   -0.10%     
==========================================
  Files           3        3              
  Lines         983     1016      +33     
==========================================
+ Hits          982     1014      +32     
- Misses          1        2       +1     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.