feat!: add variable store to embeddings registry (#2112)

BREAKING CHANGE: embedding function implementations in Node need to now
call `resolveVariables()` in their constructors and should **not**
implement `toJSON()`.

This tries to address the handling of secrets. In Node, they are
currently lost. In Python, they are currently leaked into the table
schema metadata.

This PR introduces an in-memory variable store on the function registry.
It also allows embedding function definitions to label certain config
values as "sensitive", and the preprocessing logic will raise an error
if users try to pass in hard-coded values.

Closes #2110
Closes #521

---------

Co-authored-by: Weston Pace <weston.pace@gmail.com>

docs/src/embeddings/custom_embedding_function.md

@@ -55,6 +55,14 @@ Let's implement `SentenceTransformerEmbeddings` class. All you need to do is imp
 
 This is a stripped down version of our implementation of `SentenceTransformerEmbeddings` that removes certain optimizations and default settings.
 
+!!! danger "Use sensitive keys to prevent leaking secrets"
+    To prevent leaking secrets, such as API keys, you should add any sensitive
+    parameters of an embedding function to the output of the
+    [sensitive_keys()][lancedb.embeddings.base.EmbeddingFunction.sensitive_keys] /
+    [getSensitiveKeys()](../../js/namespaces/embedding/classes/EmbeddingFunction/#getsensitivekeys)
+    method. This prevents users from accidentally instantiating the embedding
+    function with hard-coded secrets.
+
 Now you can use this embedding function to create your table schema and that's it! you can then ingest data and run queries without manually vectorizing the inputs.
 
 === "Python"