Previously we were silently setting the content-type to msgpack, now we _explicitly_ set it in the method
Also, we we previously _weren't_ setting the contentType in HttpStreamRequest, now we do

Store logs as `DatadogLogEvent`, so we don't _have_ to serialize on the main thread, and instead can defer to when we send the logs
Also update unit tests to not rely on Thread.Sleep (as makes tests flaky)

* Currently avoids calculating exact size of messages to reduce allocations, that's something we could/should look at later
* Use a shared string builder
* Currently only supports sending as JSON and Http.
* Retries up to 5 times on error, give up after ~7.5s (original sink tries 10 times, for up to 6 minutes!)
* Skip retry on 400 error
* Currently not collecting metrics about the send itself, we can add those if necessary

Accounts for the fact they may have already added the sink directly

This is slightly tricky because:
* ILoggerFactory uses a different "root" constructor in .net 5+
* We can't reverse duck type the ILoggerProvider directly, because we need to implement two interfaces, so have to `DefineType` an interface that implements both, and then duck type that.

Instruments the AppenderCollection.ToArray() method to sneak our own appender in every time a logger is requested

Added a MockLogsIntake to support verifying logs are shipped correctly, and have the expected properties

In the previous version, everything was added to a giant (persistent) StringBuilder, we then call ToString(), and convert the string to a byte array (which is then thrown away). The string and utf8 array are ultimately garbage.

In this version, we keep a smaller persistent StringBuilder (one for each log, instead of all logs), convert that to a string, and write the bytes to a persistent byte array. Only the intermediate string is garbage.

The benefit should be that we have less chance of the string/byte array ending up on the LOH, and we can reuse the UTF8 array