Add and document sqlpage.hmac function #1758

Job	Run time
test (postgres, postgres, postgres://root:[email protected]/sqlpage)	1m 56s
compile_and_lint	2m 4s
test (odbc, postgres, Driver={PostgreSQL Unicode};Server=127.0.0.1;Port=5432;Database=sqlpage;UID...	1m 56s
windows_test	2m 52s
test (mssql, mssql, mssql://root:[email protected]/sqlpage)	1m 57s
test (mysql, mysql, mysql://root:[email protected]/sqlpage)	1m 41s
docker_build (linux/arm/v7)	8m 17s
docker_build (linux/arm64)	8m 29s
docker_build (linux/amd64)	7m 43s
docker_push	0s
	36m 55s

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