feat(llmobs): add retrieval and embedding spans [backport 2.9]

[github-actions]

Backport d10e081 from #9134 to 2.9.

This PR adds support for submitting embedding and retrieval type spans for LLM Observability, both via LLMObs.{retrieval/embedding} and @ddtrace.llmobs.decorators.{retrieval/embedding}.
Additionally, this PR adds a public helper class ddtrace.llmobs.utils.Documents for users to create SDK-compatible input/output annotation objects for Embedding/Retrieval spans.

Embedding spans require a model name to be set, and also optionally accepts model provider values (will default to custom). Embedding spans can be annotated with:

• input: strings, dictionaries, or a list of dictionaries, which will be cast as Documents when submitted to LLMObs.
• output: strings or any JSON serializable value.

Retrieval spans can be annotated with:

• input strings or any JSON serializable value.
• output strings, dictionaries, or a list of dictionaries, which will be cast as Documents when submitted to LLMObs.

This PR also introduces a class of type ddtrace.llmobs.utils.Documents, which can be used to convert arguments to be tagged as input/output documents. The Documents TypedDict object can contain the following fields:

• name: str
• id: str
• text: str
• score: int/float

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy