Feature: faststream should only require opentelemetry-api

[florianmutter]

To make it easier to switch the opentelemetry implementation it should be up to the user to install an implementation. The documentation can suggest opentelemetry-sdk but it should not be installed by default. This would allow a user to replace opentelemetry-sdk with a different implementation (e.g. Datadogs ddtrace that also implements the opentelemetry-api: https://ddtrace.readthedocs.io/en/stable/api.html#opentelemetry-api)

This is also recommended by OpenTelemetry here: https://opentelemetry.io/docs/concepts/instrumentation/libraries/#opentelemetry-api