[OTel] Add "instrumenter" configuration option

[tonyo]

Extracted from #537.

In other SDKs, there's "instrumenter" configuration in ClientOptions -- we need to confirm whether we need that.

[tonyo]

"instrumenter" attribute in Sentry.Init in other SDKs (e.g. in Python, JavaScript) is basically used to control which automatic instrumentation should be used, to avoid double instrumentation in certain scenarios (for example, when we have a framework that is instrumented by both Sentry and OpenTelemetry).

Since the Sentry instrumentation we support for Go is mostly manual, we don't really need the "instrumenter" option right now: there are OpenTelemetry instrumentation libraries for a much bigger number of libraries/frameworks, so people should just use those (if they already use OTel), and Sentry-OTel bridge will take care of everything else.