feat: add stored property for adding additional headers in SdkHttpRequest

[dayaffe]

Provide the ability to add additional headers to an existing request of type SdkHttpRequest.

Issue #

#1130
#583

Description of changes

Cause: headers is already a mutable property but it is computed without any stored values. This means that a customer who attempts to modify headers will either have no effect (because computed properties are read-only) or because it points to an instance of endpoint.headers whose headers are immutable.

Solution: Add additionalHeaders stored property which is mutable and provide add/removal methods.

Risk: Computed properties ensure that the request is never modified in-transit. In order to reduce any risk associated I have chosen to add a new property that is mutable, stores values, and provided my own setters rather than modify the existing property to no longer be computed. Additionally, we should remove this functionality once Swift SDK SRA-compliant with interceptors (#307)

How to use:

class MyHTTPClientEngineProxy: HttpClientEngine {
    let base: HttpClientEngine

    init(base: HttpClientEngine) {
        self.base = base
    }

    func execute(request: SdkHttpRequest) async throws -> HttpResponse {
        request.withHeader(name: "foo", value: "bar")
        return try await base.execute(request: request)
    }
}

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sichanyoo]

Lgtm

[jbelkins]

I approve this, as an interim fix to restore what Amplify needs, but this should be removed once reference architecture features that allow for headers to be set are implemented.

[dayaffe]

This PR should be reverted once #307 is implemented.