feat: allow callbacks as strings

[tlambert03]

        Action(
            id='some.command',
            title="Some Title",
            callback="my_module:some_function",  # resolve at calltime please
        ),

[codecov]

Codecov Report

Merging #18 (4be3b1c) into main (2924c02) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
+ Coverage   99.47%   99.63%   +0.15%     
==========================================
  Files          27       28       +1     
  Lines        1339     1374      +35     
==========================================
+ Hits         1332     1369      +37     
+ Misses          7        5       -2     

Impacted Files	Coverage Δ
src/app_model/registries/_commands_reg.py	96.87% <100.00%> (+4.42%)	⬆️
src/app_model/types/_action.py	100.00% <100.00%> (ø)
src/app_model/types/_utils.py	100.00% <100.00%> (ø)

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