feat: Expose process_index context variable for server processes

[achimnol]

This PR adds a context variable process_idex to the aiotools.server module so that the worker processes could read the index of current process anywhere.

[codecov]

Codecov Report

Merging #31 (1213aa3) into main (3a65b38) will decrease coverage by 0.12%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
- Coverage   77.38%   77.26%   -0.13%     
==========================================
  Files          10       10              
  Lines         849      862      +13     
==========================================
+ Hits          657      666       +9     
- Misses        192      196       +4     

Impacted Files	Coverage Δ
src/aiotools/server.py	50.17% <71.42%> (+0.90%)	⬆️

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