Enable automation of data acquisition through the MQTT API

[ethanjli]

We should add the following automation functionalities:

• Add a basic imager/image "image" command with a hard-coded set of minimal metadata (e.g. just use the timestamp as the acquisition ID, with no ability to use anything else as the acquisition ID) (Expose interfaces for stop-flow imaging #272)
• Enable embedding of timestamps from HCL's Go templates in strings, to use for acquisition IDs
• Enable arbitrary acquisition IDs including templated substitutions
• Send an imager/image "update_config" command with an arbitrary JSON payload as the command's "config" parameter, including required IDs.