API to define a number of workers

[ahopkins]

This PR adds a relatively small change to add a workers param to the manager so that it is easier to spin up multiple custom workers.

@app.main_process_ready
async def ready(app: Sanic, _):
    app.manager.manage(..., workers=3)

[codecov]

Codecov Report

Patch coverage: 100.000% and project coverage change: +0.106 🎉

Comparison is base (88c918e) 88.539% compared to head (e266d66) 88.646%.

❗ Current head e266d66 differs from pull request most recent head e050111. Consider uploading reports for the commit e050111 to get more accurate results

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2701       +/-   ##
=============================================
+ Coverage   88.539%   88.646%   +0.106%     
=============================================
  Files           87        87               
  Lines         6815      6817        +2     
  Branches      1171      1172        +1     
=============================================
+ Hits          6034      6043        +9     
+ Misses         537       533        -4     
+ Partials       244       241        -3     

Impacted Files	Coverage Δ
sanic/worker/manager.py	94.047% <100.000%> (ø)
sanic/worker/process.py	89.285% <100.000%> (+0.194%)	⬆️

... and 2 files with indirect coverage changes

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[Tronic]

LGTM