refactor!: simplify stac-api structures #629

Job	Run time
test-ubuntu-with-pgstac	35s
test-ubuntu (-p stac)	23s
doc	1m 14s
test-ubuntu (-p stac -F reqwest)	1m 24s
build	51s
fmt	12s
test-ubuntu (-p stac-api)	17s
test-ubuntu (-p stac -p stac-api -F geo)	47s
test-macos	43s
test-ubuntu-with-gdal	1m 7s
test-windows	3m 13s
test-ubuntu (-p stac-async)	52s
test-ubuntu (-p stac-cli --no-default-features)	1m 3s
test-ubuntu (-p stac-validate)	1m 9s
	13m 50s

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