feat(php): undiscriminated unions #910

Job	Run time
cli-validate	1m 57s
generator-validate (fastapi)	2m 10s
generator-validate (pydantic)	1m 55s
generator-validate (python-sdk)	1m 57s
generator-validate (go-sdk)	1m 58s
generator-validate (ruby-sdk)	1m 55s
generator-validate (postman)	2m 5s
generator-validate (openapi)	1m 57s
generator-validate (java-sdk)	1m 56s
generator-validate (java-model)	2m 6s
generator-validate (java-spring)	3m 17s
generator-validate (ts-express)	1m 58s
	25m 11s

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