Move GenAI common utils into opentelemetry-instrumentation package

[aabmass]

So that they can be used across those packages. OpenAI, Cohere, and Vertex instrumentations need some common utils. See https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3081/files/#r1884751133

They will be available in the opentelemetry.instrumentation.gen_ai module.

Because GenAI packages are not released in lockstep with the rest of the repo, we will need to first release this change before updating those instrumentation packages to use it. See this comment #3188 (comment)

[aabmass]

Discussed in Python SIG. Given there are only a few utils, I think we might table this PR until we have more need for code sharing.

It would be better to put it into a new shared package opentelemetry-util-gen-ai which is also released independently. That would help us move faster.