Add and document sqlpage.hmac function #1751

Job	Run time
test (odbc, postgres, Driver={PostgreSQL Unicode};Server=127.0.0.1;Port=5432;Database=sqlpage;UID...	4m 13s
test (postgres, postgres, postgres://root:[email protected]/sqlpage)	2m 47s
test (mysql, mysql, mysql://root:[email protected]/sqlpage)	2m 5s
compile_and_lint	2m 44s
docker_build (linux/amd64)	7m 46s
windows_test	5m 33s
docker_build (linux/arm/v7)	8m 16s
test (mssql, mssql, mssql://root:[email protected]/sqlpage)	2m 35s
docker_build (linux/arm64)	8m 32s
docker_push	0s
	44m 31s

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