Add a way to pass custom headers directly to the request [SDK-2908]

[adamjmcgrath]

Changes

Add the headers(_: [String: String]) method to the Request type, which allows the developer to pass arbitrary HTTP headers to a given request.

Auth0
    .authentication()
    .renew(withRefreshToken: refreshToken)
    .headers(["key": "value"])
    .start { result in
        switch result {
        case .success(let credentials): dump(credentials)
        case .failure(let error): print(error)
        }
    }

added a headers: [String: String] = [:] parameter to the credentials(withScope:minTTL:parameters:callback:) and the revoke(:_) public methods of the Credentials Manager which use the new headers(_: [String: String]) method to add the headers to the Request.

Testing

• This change adds unit test coverage
• This change has been tested on the latest version of the platform/language

Checklist

• I have read the Auth0 general contribution guidelines
• All existing and new tests complete without errors
• All active GitHub checks have passed

[Widcket]

Looking good, left a few comments.

[Widcket]

LGTM