Move OTel code out of `integrations/`

[sentrivana]

Right now, POTel is written as an integration. This is problematic since it can be disabled via default_integrations=False. Also in architectural sense it's no longer an integration but an integral part of the SDK, so it's time for it to move out of the integrations/ directory.

This should also enable us to clean up the code and get rid of some inline imports due to circular imports.

Consider not breaking people importing the processor/propagator by reexporting it in the old place. Also add a message, maybe.

• Move scope context init outside integration #3850
• ref: Move sampler out of integrations/opentelemetry/ #4258
• ref: Move span processor out of integrations/opentelemetry/ #4259
• ref: Move propagator out of integrations/opentelemetry/ #4260
• ref: Move contextvars_context out of integrations #4271
• ref: Move consts, utils from integrations/opentelemetry/ #4272
• ref: Move otel scope out of integrations/opentelemetry/ #4276
• ref: Move OTel setup out of integrations/opentelemetry/ #4277