BackpressureMonitor should be a constructor dependency of HttpTransportBase

[jamescrosswell]

Inject an optional BackpressureMonitor into the HttpTransportBase via the constructor (rather than storing/retrieving this from the options).

Originally posted by @Flash0ver in #4452 (comment)

[linear]

NET-452 question: taking the breaking change in 6.0?

[Flash0ver]

In the original PR, we added a new .ctor:

internal HttpTransportBase(SentryOptions, BackpressureMonitor, Func<string, string> getEnvironmentVariable, ISystemClock? clock)

We could either remove this .ctor again and add the BackpressureMonitor parameter to the protected .ctor,
or we overload the protected .ctor with another protected .ctor.

I guess the former is more "discoverable" and the breaking change acceptable for the major release.

Alternatively, if we are happy with the current solution (as the internal .ctor was added after this issue has been created), we may close this issue as obsolete.