Document ingress_per_unit lib dependencies

[mmanciop]

Issue

Document the dependency of ingress_per_unit on the jsonschema package (which was transitively
imported by SDI with LIBPATCH 6 and below).

Solution

Provide more informative error message on the missing jsonschema dependency for charms using the ingress_per_unit library.

Some housekeeping on requirements.txt.

Context

N/A

Testing Instructions

N/A

Release Notes

Document the dependency of ingress_per_unit on the jsonschema package (which was transitively
imported by SDI with LIBPATCH 6 and below).

[github-actions]

Libraries are not up to date with their remote counterparts. If this was
not intentional, run charmcraft fetch-libs and commit the updated libs
to your PR branch.

stdout

Library charms.observability_libs.v0.kubernetes_service_patch was already up to date in version 0.6.                                                           
Library charms.prometheus_k8s.v0.prometheus_scrape was already up to date in version 0.17.                                                                     
Library charms.traefik_k8s.v0.ingress was already up to date in version 0.5.   
Library charms.traefik_k8s.v0.ingress_per_unit has local changes, cannot be updated.                                                                           

stderr

[jnsgruk]

@PietroPasotti let's get this merged in once the other has landed.

[PietroPasotti]

LGTM