Refactor EmbeddingConfig

[ratzrattillo]

Currently, the EmbeddingConfig is limited to several hardcoded Providers like the ones listed under https://docs.unstructured.io/api-reference/ingest/ingest-configuration/embedding-configuration#configs

There is no way a user can extend this list and create its own EmbeddingConfig for e.g. an Ollama instance.

As a user, i would prefer to be able to hand over a custom Encoder to an instance of a LocalRunner.

What do you think?

[ratzrattillo]

Seems like there is already work being done for Ollama in #288 and #153

Still, the interface is not the most easy to extend, if users want to add their own Embedders!