Add and document sqlpage.hmac function #1759

Job	Run time
docker_build (linux/arm/v7)	8m 35s
docker_build (linux/amd64)	7m 47s
compile_and_lint	1m 55s
windows_test	4m 8s
docker_build (linux/arm64)	8m 36s
test (odbc, postgres, Driver={PostgreSQL Unicode};Server=127.0.0.1;Port=5432;Database=sqlpage;UID...	2m 36s
test (mssql, mssql, mssql://root:[email protected]/sqlpage)	2m 31s
test (postgres, postgres, postgres://root:[email protected]/sqlpage)	2m 12s
test (mysql, mysql, mysql://root:[email protected]/sqlpage)	1m 49s
docker_push	0s
	40m 9s

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