feat: Isaacus embeddings integration (#20124)

* feat: add Isaacus embeddings integration

* Add authorship information

* style: apply linter formatting fixes

* docs: add sample notebook and API reference .md

* fix: move mid-code imports to top level

* fix: no need for ImportError checks

* fix: enable Pydantic to properly validate the Literal types at runtime

Author: adlumal

docs/api_reference/api_reference/embeddings/isaacus.md

@@ -0,0 +1,3 @@
+::: llama_index.embeddings.isaacus
+options:
+members: - IsaacusEmbedding