Expose AnySerializer via core schema

[BoxyUwU]

Change Summary

Fixes #1245

Allows simple_ser_schema to specify 'any' so that AnySerializer can be used when creating core schemas. The test_any.py file seems to adequately test the actual implementation of AnySerializer so I've only added tests that check that simple_ser_schema('any') actually results in an AnySerializer rather than duplicating the entire file 😅

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1394 will not alter performance

_{Comparing boxy/any_ser_schema (bdefd96) with main (f79d49b)}

Summary

✅ 155 untouched benchmarks

[davidhewitt]

Looks great, thanks very much!