Adopt explicit existential any

[czechboy0]

Motivation

As a follow-on to apple/swift-openapi-generator#99, let's also enable explicit existential any in the runtime library.

Modifications

Enabled the flag in Package.swift (for Swift 5.9 and newer) and updated all the usage sites.

Result

The codebase is closer to being ready for Swift 6.

Test Plan

Updated unit tests as well, all is passing locally.

[czechboy0]

Please ignore API breakage, known to fail when making changes like this, but safe to land:

10 breaking changes detected in OpenAPIRuntime:
  💔 API breakage: var OpenAPIValueContainer.value has declared type change from Swift.Sendable? to (Swift.Sendable)?
  💔 API breakage: accessor OpenAPIValueContainer.value.Get() has return type change from Swift.Sendable? to (Swift.Sendable)?
  💔 API breakage: accessor OpenAPIValueContainer.value.Set() has parameter 0 type change from Swift.Sendable? to (Swift.Sendable)?
  💔 API breakage: var OpenAPIObjectContainer.value has declared type change from [Swift.String : Swift.Sendable?] to [Swift.String : (Swift.Sendable)?]
  💔 API breakage: accessor OpenAPIObjectContainer.value.Get() has return type change from [Swift.String : Swift.Sendable?] to [Swift.String : (Swift.Sendable)?]
  💔 API breakage: accessor OpenAPIObjectContainer.value.Set() has parameter 0 type change from [Swift.String : Swift.Sendable?] to [Swift.String : (Swift.Sendable)?]
  💔 API breakage: var OpenAPIArrayContainer.value has declared type change from [Swift.Sendable?] to [(Swift.Sendable)?]
  💔 API breakage: accessor OpenAPIArrayContainer.value.Get() has return type change from [Swift.Sendable?] to [(Swift.Sendable)?]
  💔 API breakage: accessor OpenAPIArrayContainer.value.Set() has parameter 0 type change from [Swift.Sendable?] to [(Swift.Sendable)?]
  💔 API breakage: constructor ServerError.init(operationID:request:requestMetadata:operationInput:operationOutput:underlyingError:) has parameter 5 type change from Swift.Error to (Swift.Error)

[simonjbeaumont]

[czechboy0]

@swift-server-bot test this please

[czechboy0]

The changes to the code are good, but not adding the enforcement yet until apple/swift-openapi-generator#119

[czechboy0]

Ok Integration Test passes again.

[simonjbeaumont]

Rather than just disable it everywhere, let's enable it in just the Swift 5.9+ builds, by adding -Xswiftc -enable-upcoming-feature -Xswiftc ExistentialAny to the 5.9 and nightly docker-compose files.

This way we won't regress.