Add and document sqlpage.hmac function #1755

Job	Run time
compile_and_lint	1m 42s
windows_test	2m 43s
test (mssql, mssql, mssql://root:[email protected]/sqlpage)	2m 43s
test (odbc, postgres, Driver={PostgreSQL Unicode};Server=127.0.0.1;Port=5432;Database=sqlpage;UID...	2m 27s
test (postgres, postgres, postgres://root:[email protected]/sqlpage)	2m 27s
test (mysql, mysql, mysql://root:[email protected]/sqlpage)	1m 53s
docker_build (linux/amd64)	7m 45s
docker_build (linux/arm/v7)	9m 31s
docker_build (linux/arm64)	8m 51s
docker_push	0s
	40m 2s

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