Add HttpTransport extensibility and synchronous serialization support

[mattjohnsonpint]

As mentioned in #1553 (comment), we need to have a clean way for higher-level SDKs (like sentry-unity) to implement their own HTTP transports. In doing so, we'll need to support both asynchronous and synchronous handling of response messages, including synchronous serialization support for things like logging of payloads.

This PR takes most of the functionality of HttpTransport and pulls it into an abstract base class, HttpTransportBase. That allows protected access to the core functionality of the transport, reducing SendEnvelopeAsync to a small bit of orchestration code. Higher-level SDKs (such as UnityWebRequestTransport proposed in getsentry/sentry-unity#657) can implement their own version of that method without having to take a dependency on HttpClient or async/await.

Note that ITransport is still internal and implemented only on HttpTransport - not HttpTransportBase.

A fully synchronous path for handling response messages is available as HandleResponse instead of HandleResponseAsync (either can be used by the parent class). In order to fully implement this correctly, Sentry.Protocol.Envelopes.ISerializable has been given a Serialize method which mirrors the existing SerializeAsync method. All implementations and tests are identical, except as to whether async methods are employed or not. At the lowest level, these now use things like Stream.Write instead of Stream.WriteAsync - which are implemented in .NET using the correct primitives (no sync over async or vice versa).

[mattjohnsonpint]

Just because it's a bit hard to read in the diff, here's the new version of HttpTransport.SendEnvelopeAsync in its entirety. This is now the only method in the class.

public async Task SendEnvelopeAsync(Envelope envelope, CancellationToken cancellationToken = default)
{
    using var processedEnvelope = ProcessEnvelope(envelope);
    if (processedEnvelope.Items.Count > 0)
    {
        using var request = CreateRequest(processedEnvelope);
        using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);
        await HandleResponseAsync(response, processedEnvelope, cancellationToken).ConfigureAwait(false);
    }
}

[vaind]

Works fine for the unity WebGL implementation. There's just one open point that could maybe be improved. See getsentry/sentry-unity#657 (review)

[bruno-garcia]

LGTM

[mattjohnsonpint]

I removed the HttpContentReader, and just added a synchronous serialization path for EnvelopeHttpContent.