Allow jinja2 template task definitions

[adammcdonagh]

• Add mode override docs to --help
• Allow task definitions to use a whole templated object within them. e.g a protocol definition could be defined as a global variable to define SFTP connectivity to a commonly used source/destination. This is done using .j2 task definition file instead of .json

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.06% 🎉

Comparison is base (8aed192) 87.95% compared to head (b449578) 88.02%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   87.95%   88.02%   +0.06%     
==========================================
  Files          17       17              
  Lines        1968     1970       +2     
==========================================
+ Hits         1731     1734       +3     
+ Misses        237      236       -1     

Files Changed	Coverage Δ
src/opentaskpy/config/loader.py	98.44% <100.00%> (+0.02%)	⬆️

... and 1 file with indirect coverage changes

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