Add "system_remote_item_url" and "system_remote_tracker_id" WIT fields

[kwk]

This adds two non-required new fields to all work item types (by adding it to the Planner Item WIT from which all current WITs derive).

    "system_remote_item_url":
      label: Remote URL
      description: URL to remote work item
      type:
        kind: url
      required: no
    "system_remote_tracker_id":
      label: Remote Tracker
      description: Reference to the tracker that manages this work item
      type:
        kind: remotetracker
      required: no

They will be used by @DhritiShikhar in her work on the remote work item tracking.

Please note that the field system_remote_tracker_id uses a kind of type remotetracker. That is a relational kind which is the first one that will not accept any string on input but only a UUID (as a string or as a uuid.UUID object).

[codecov[bot]]

Codecov Report

Merging #2366 into master will increase coverage by 2.27%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2366      +/-   ##
=========================================
+ Coverage   67.63%   69.9%   +2.27%     
=========================================
  Files         182     171      -11     
  Lines       17797   17083     -714     
=========================================
- Hits        12037   11942      -95     
+ Misses       4596    3971     -625     
- Partials     1164    1170       +6

Impacted Files	Coverage Δ
workitem/field_definition.go	82.9% <100%> (ø)	:arrow_up:
workitem/field_test_data.go	95.83% <100%> (+0.41%)	:arrow_up:
workitem/simple_type.go	76.92% <100%> (+1.59%)	:arrow_up:
migration/migration.go	69.28% <0%> (-8.58%)	:arrow_down:
remoteworkitem/trackerquery_repository.go	58.97% <0%> (-3.99%)	:arrow_down:
controller/trackerquery.go	56.32% <0%> (-1.18%)	:arrow_down:
controller/deployments_osioclient.go	90.24% <0%> (ø)	:arrow_up:
goasupport/conditional_request/generator.go
gormsupport/cleaner/db_clean.go
resource/require.go
... and 8 more

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[DhritiShikhar]

@kwk we might need to introduce other fields as well - probably to track the active/detached state etc.

For now, this looks good. If any more fields are required for remoteworkitem feature, we could raise another PR for that.