extended help for beacons

[erwindon]

Is your feature request related to a problem? Please describe.
Creating beacons is a challenge because the exact format of the parameters depends on the beacon that is used.
The regular help facility only mentions the detail parameters as yaml text, while we expect json.

Describe the solution you'd like
Create a "command constructor" that helps to fill in placeholders.
e.g. start with "beacons.add <name> <data>"
the command field should get a drop-down with common combinations.

Describe alternatives you've considered
provide this information via the help text

Additional context
We likely cannot detect plugged in beacons, let alone their parameter structure.

This help-structure must also be useable for other commands

[erwindon]

function beacons.list_available exists. this helps to divide the help-text 3 parts:

• known beacons that we can use
• known beacons that we cannot use
• unknown beacons that are reported as available (and possibly in use)

we have no help information for the 3rd category.

we assume that all minions have roughly the same 'beacon' capabilities. we request the list of known beacons from all minions once. then we present the help text as if all minions support all these beacons.