Provide get_workchain_metadata help function

[superstar54]

This PR adds another help function get_workchain_metadata function to tell the app how many calcjobs (estimation) will be submitted in the workchain. This is useful to show the progress in the workflow.

Note: we have many help functions in the workchain entry, and it would be better to provide a class to wrap all of them together in the future.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.98%. Comparing base (6228414) to head (e9a5579).

Files with missing lines	Patch %	Lines
src/aiidalab_qe/plugins/bands/workchain.py	66.66%	1 Missing ⚠️
src/aiidalab_qe/plugins/pdos/workchain.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1173      +/-   ##
==========================================
- Coverage   72.99%   72.98%   -0.01%     
==========================================
  Files          94       94              
  Lines        6532     6538       +6     
==========================================
+ Hits         4768     4772       +4     
- Misses       1764     1766       +2     

Flag	Coverage Δ
python-3.11	72.97% <66.66%> (-0.03%)	⬇️
python-3.9	72.99% <66.66%> (+<0.01%)	⬆️

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