Refactor stac-api by simplifing structures #627

Job	Run time
test-ubuntu-with-gdal	1m 11s
test-ubuntu-with-pgstac	39s
build	45s
doc	1m 12s
fmt	5s
test-ubuntu (-p stac)	23s
test-ubuntu (-p stac -F reqwest)	1m 17s
test-windows	3m 19s
test-macos	43s
test-ubuntu (-p stac-api)	20s
test-ubuntu (-p stac -p stac-api -F geo)	41s
test-ubuntu (-p stac-async)	1m 0s
test-ubuntu (-p stac-cli --no-default-features)	51s
test-ubuntu (-p stac-validate)	1m 8s
	13m 34s

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