Add lifespan middleware

[RobbeSneyders]

This PR adds a new middleware to handle lifespan events.

I added this as a separate middleware so it is encapsulated and aligned for both the FlaskApp and AsyncApp. It leverages a Starlette Router to register and call the lifespan handler.

[coveralls]

Pull Request Test Coverage Report for Build 4410472221

• 25 of 25 (100.0%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 93.398%

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Totals
Change from base Build 4410156717:	0.03%
Covered Lines:	3296
Relevant Lines:	3529

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💛 - Coveralls

[Ruwann]

What is the reason for treating this separately from the other middlewares? Why not make it part of the list of middlewares?

[RobbeSneyders]

It is part of the list of middlewares. But you still pass in a handler which the middleware will use.

The biggest difference between the LifespanMiddleware and the other middlewares, is that this one is configured on the App level, instead of the API level. This is why the arguments are passed when instantiating the middleware instead of via the add_api method.

It would have been great to align this with the other middlewares, but Starlette currently doesn't support lifespans for nested Apps / Routers, which is the equivalent of our APIs.