Add convenience for dynamic changes to routing

[ahopkins]

When trying to add something in event listeners, you might run up against the routers having been finalized. This creates a convenience that allows dynamic changes in a slightly easier way.

@app.before_server_start
async def dynamic_router_changes(app: Sanic) -> None:
    app.add_route(...)
    app.register_middleware(...)

The above will not work in current version since routes, signals, and middleware have all been finalized. The change allows these runtime changes to occur implicitly.

Closes #2699

---

TODO

• Tests

[codecov]

Codecov Report

Patch coverage: 100.000% and project coverage change: +0.029 🎉

Comparison is base (5ee36fd) 88.607% compared to head (2f8c05a) 88.637%.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2704       +/-   ##
=============================================
+ Coverage   88.607%   88.637%   +0.029%     
=============================================
  Files           87        87               
  Lines         6820      6838       +18     
  Branches      1171      1176        +5     
=============================================
+ Hits          6043      6061       +18     
  Misses         535       535               
  Partials       242       242               

Impacted Files	Coverage Δ
sanic/app.py	90.068% <100.000%> (+0.315%)	⬆️

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