Add set variable function and update docs #2012

Job	Run time
windows_test	3m 17s
compile_and_lint	1m 32s
docker_build (linux/amd64)	8m 2s
test (mssql, mssql, mssql://root:[email protected]/sqlpage)	1m 42s
test (mysql, mysql, mysql://root:[email protected]/sqlpage)	1m 35s
docker_build (linux/arm64)	9m 16s
docker_build (linux/arm/v7)	9m 11s
test (postgres, postgres, postgres://root:[email protected]/sqlpage)	2m 15s
test (odbc, postgres, Driver=PostgreSQL Unicode;Server=127.0.0.1;Port=5432;Database=sqlpage;UID=r...	2m 6s
docker_push	0s
	38m 56s

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