feat: Add `SerializationPlugin` and `InitPlugin` to replace their respective protocols #4848

Job	Run time
mypy	1m 4s
pyright	38s
slotscheck	23s
validate	24s
Test server integration (uvicorn<0.27.0)	40s
Test Minimal Application with Base Dependencies	13s
Test server integration (uvicorn>=0.27.0)	39s
Test Minimal Pydantic 1 application	16s
test-platform-compat	0s
test (3.10) / test	1m 52s
test (3.11) / test	1m 51s
test (3.12) / test	3m 0s
test (3.13) / test	1m 57s
test (3.8) / test	2m 51s
test (3.9) / test	2m 7s
build-docs	2m 20s
codeql	2m 19s
sonar	59s
upload-test-coverage	18s
test_success	1s
	23m 52s

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