fix(process): ensure layoutset is added when adding signing task

[nkylstad]

Description

As per the documentation of the signing task, a layoutset needs to be associated with the task.
Updated the handlers that handle adding tasks to the process to add a layoutset for the signing task.
Also ensure that the RecommendedNextAction for naming layoutset is shown in this case.

Future: We could automatically add the signing button, in the same way that we add the payment component for payment layoutset. However, I am leaving that to a later PR.

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.13%. Comparing base (dae7a6b) to head (b347945).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...packages/process-editor/src/hooks/useBpmnEditor.ts	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13934   +/-   ##
=======================================
  Coverage   95.13%   95.13%           
=======================================
  Files        1673     1673           
  Lines       22189    22191    +2     
  Branches     2608     2609    +1     
=======================================
+ Hits        21109    21111    +2     
  Misses        834      834           
  Partials      246      246           

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[nkylstad]

Tested this manually in dev.