Prepare being compatible with runtime 0.2.0 release (#12)

Prepare being compatible with runtime 0.2.0 release

### Motivation

Runtime 0.2.0 is compatible with 0.1.3+ in the APIs adopted in the transport, so just reflect that in the manifest.

### Modifications

Increased the supported runtime range to include 0.2.x.

### Result

Once released, folks will be able to use 0.1.x of this transport with both 0.1.x and 0.2.x of runtime.

### Test Plan

Will be manually tested once runtime 0.2.0 is released.


Reviewed by: gjcairo

Builds:
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#12

Adopt the new shared HTTP client (#13)

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

Disable ExistentialAny temporarily (#9)

Disable ExistentialAny temporarily

### Motivation

Unfortunately until we adopt 5.9, adding ExistentialAny on upstream packages has unintended consequences for some downstream packages, so disabling for now. Details in apple/swift-openapi-generator#120

### Modifications

Disabled the feature enforcement, but the code changes are there, so downstream adopters can still use them.

### Result

We won't be seeing the issue described in apple/swift-openapi-generator#120.

### Test Plan

PR CI, which discovered the original issue.


Reviewed by: 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. 

#9

Relax deployment target to macOS 10.15 (#6)

### Motivation

Relax deployment target to macOS 10.15

### Modifications

- Reduced requirement from macOS 13 to macOS 10.15
- Added the minimum version to readme and documentation

### Result

- Users will be able to deploy it on projects with a minimum deployment
target of 10.15

Initial commit

Co-authored-by: Honza Dvorsky <honza@apple.com>
Co-authored-by: Si Beaumont <beaumont@apple.com>
Co-authored-by: David Nadoba <d_nadoba@apple.com>