[ENH] Facilitate Where and Rank construction (#5659)

## Description of changes

_Summarize the changes made by this PR._

- Improvements & Bug fixes
- Facilitate `Where` and `Rank` construction with operator overrides and
builder pattern
  - Cleanups
- Drop default KNN limit from 128 to 16, which is consistent with python
client
    - Update type of  `RankExpr::Knn.key` to `Key` 
- New functionality
  - N/A

## Test plan

_How are these changes tested?_

- [ ] Tests pass locally with `pytest` for python, `yarn test` for js,
`cargo test` for rust

## Migration plan

_Are there any migrations, or any forwards/backwards compatibility
changes needed in order to make sure this change deploys reliably?_

## Observability plan

_What is the plan to instrument and monitor this change?_

## Documentation Changes

_Are all docstrings for user-facing APIs updated if required? Do we need
to make documentation changes in the [docs
section](https://github.com/chroma-core/chroma/tree/main/docs/docs.trychroma.com)?_

Author: Sicheng-Pan

rust/frontend/src/impls/service_based_frontend.rs

@@ -1723,7 +1723,10 @@ impl ServiceBasedFrontend {
                         let knn_queries = rank_expr.knn_queries();
                         for knn_query in knn_queries {
                             schema
-                                .is_knn_key_indexing_enabled(&knn_query.key, &knn_query.query)
+                                .is_knn_key_indexing_enabled(
+                                    &knn_query.key.to_string(),
+                                    &knn_query.query,
+                                )
                                 .map_err(|err| {
                                     QueryError::Other(Box::new(err) as Box<dyn ChromaError>)
                                 })?;

rust/types/src/execution/error.rs

@@ -22,12 +22,18 @@ pub enum QueryConversionError {
     Where(#[from] WhereConversionError),
     #[error("Error parsing scan: {0}")]
     Scan(#[from] ScanToProtoError),
+    #[error("Validation error: {0}")]
+    Validation(String),
 }
 
 impl QueryConversionError {
     pub fn field(name: impl ToString) -> Self {
         Self::Field(name.to_string())
     }
+
+    pub fn validation(msg: impl ToString) -> Self {
+        Self::Validation(msg.to_string())
+    }
 }
 
 impl From<QueryConversionError> for Status {