Allow for simple signals

[ahopkins]

The point of this PR is to allow for definition and use of simple signals. Sometimes, you do not need the full three part scheme.

@app.signal("<thing>")
async def handler(**kwargs):
    print("foobar signal received")
    print(kwargs)


@app.route("/")
async def test(request: Request):
    await request.app.dispatch("foobar")
    return json({"hello": "world"})

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7d93c10) 88.348% compared to head (02314fb) 88.244%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2813       +/-   ##
=============================================
- Coverage   88.348%   88.244%   -0.105%     
=============================================
  Files           92        92               
  Lines         7175      7188       +13     
  Branches      1232      1234        +2     
=============================================
+ Hits          6339      6343        +4     
- Misses         581       589        +8     
- Partials       255       256        +1     

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