embeddings: use NewEmbedder for OpenAI embeddings

[eliben]

This is the first use of the refactoring made in #374 -- removing the embeddings/openai directory entirely, since it can be now implemented with NewEmbedder without duplicating code. The test was moved one dir down and works with the new setup.

This is a model that can be used to similarly refactor the other embeddings/* subdirs.

This is a large PR because it also updates example go.mods

For #379

PR Checklist

• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.

[eliben]

@tmc for your review

@zivkovicn FYI

[tmc]

👏 LGTM

[nidzola]

@eliben nice work! Tnx