cron rpc action

[Alan Noble]

As prerequisite for Issue #281, add support for rpc as a cron action.

Analogous to the call action, the “variable” name will specify the rpc endpoint (rather than a function name). The endpoint will be invoked via a HTTP POST method request, with (optional) data in the request body.

[Alan Noble]

Implemented in https://bitbucket.org/ausocean/vidgrind/commits/95807120f34a52af4b40c1a4aee0007733bbfdff