Create a base AIM subscriber to abstract handling `Llm*` events

[amychisholm03]

Description

We have begun to move a few AIM packages over to subscriber-based, namely openai and @google/genai (at the time of writing). They reuse similar logic; it would be useful to have a base class that encapsulates this behavior.

Some features of this new base AIM subscriber:

• Subscriber.enabled is set to super.enabled && this.agent?.config?.ai_monitoring?.enabled or super.enabled && this.agent?.config?.ai_monitoring?.streaming?.enabled for streams
• Same function names, like instrumentStream, addLlmMeta, recordEvent, and recordChatCompletionMessages. These can be defined in the base AIM subscriber as no-ops and then implemented in its children classes (e.g. OpenAISubscriber).
• handler(data,ctx) for a chat completions subscriber typically just creates a segment with name AI_PREFIX.COMPLETION. The base AIM subscriber could default to this behavior.
• Similarly for AI_PREFIX.EMBEDDING too
• Previous instrumentation checked against agent.config for the ai_monitoring value. Most subscribers just use config because they don't expect agent.config to change at runtime. I would suggest a getter for config that equates to agent.config to avoid this potential oversight.

Acceptance Criteria

• A base subscriber for AIM is created that standardizes how we handle AI packages in subscribers.
• All instances of agent.config are replaced with config to avoid redundancy
• Relevant unit tests are created

[Newrelic-Boomi]

https://new-relic.atlassian.net/browse/NR-484171

[amychisholm03]

I will wait until both #3467 and #3493 are merged to work on this as we will have converted three AIM packages over at that point.