Add entry point for opentelemetry propagator

[mender]

Problem Statement

According to the documentation, the recommended way to integrate with OpenTelemetry is to completely replace existing propagators using

set_global_textmap(SentryPropagator())

In case of distributed tracing it is not correct behaviour. We have to add the SentryPropagator to the existing ones instead. Opentelemetry provides environment variables as a most common way to configure it. However, propagators added via environment variables must be declared in entry points (ref). And here is an example of how entry points are declared in OpenTelemetry.

Solution Brainstorm

I propose adding entry point for SentryPropagator in setup.py.

[sentrivana]

Thank you for the issue and the PR @mender, we will take a look.