[BUG] TextEmbeddingPipeline missing skip_existing property

### What is the bug?
As per https://docs.opensearch.org/latest/ingest-pipelines/processors/text-embedding/, there is a `skip_existing` property inside the text embedding model which is not defined in the types.

### How can one reproduce the bug?
Define a text_embedding pipeline with the skip_existing property

### What is the expected behavior?
No type errors