feat(guest): allow to add multiple at once

[dargmuesli]

Right now there is only an automatically generated endpoint for invitation creation that creates a single invitation at once. Inviting many guests needs many calls to that endpoint which is inefficient and might even lock users out for some time due to rate limiting. There should be a new function that expects an array of contact ids to be invited and which tries to create an invitation for all of the given contact ids at once.

Frontend issue:

• perf(invite): one request for multiple invitation creations vibetype#1654

[dargmuesli]

cc @sthelemann, to be done after internal tasks are completed.

[dargmuesli]

Make sure to check if there is native support for such api functionality in Postgrahile before implementing an own wrapper!
See: https://postgraphile.org/postgraphile/current/function-gallery#insert-multiple-records