[AHCTransport] Consistent style for initializing local variables

### Motivation

Move to a consistent style when initializing local variables, always use `let foo = Foo(...)` vs `let foo: Foo = .init(...)`.

### Modifications

Updated all occurrences of the latter to use the former.

### Result

Consistent local variable initialization.

### Test Plan

All tests passed.


Reviewed by: gjcairo, simonjbeaumont

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#10

Enable strict concurrency checking in CI

### Motivation

To further avoid concurrency bugs, enable complete concurrency checking in CI.

### Modifications

Added the compiler flag to the docker-compose scripts.

### Result

If a warning of this nature comes up, because we have warnings-as-errors, it'll fail CI.

### Test Plan

Locally built without any warnings with the flag enabled.


Reviewed by: glbrntt

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#11

Adopt the new shared HTTP client

### Motivation

Now that SwiftNIO/AsyncHTTPClient have a singleton variant of the `EventLoopGroup`, which allows creating an `HTTPClient` without any argument, let's simplify the initializer of the transport to take advantage of it - bringing it in line with the URLSession transport.

### Modifications

Default the HTTPClient to a new one with a default event loop group, and remove the mandatory shutdown call.

### Result

Adopters can more easily create the AHC transport.

### Test Plan

N/A


Reviewed by: dnadoba, glbrntt

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#13