modify functions to allow frontend to pass percentiles (#434)

## Summary
This PR allows the frontend to specify which percentiles it retrieves
from the backend. The percentiles can be passed as a query parameter:

```
percentiles=p0,p10,p90
```
If omitted, the default percentiles are used:  
```
percentiles=p5,p50,p95
```

### Example Requests *(App must be running)*  

#### Default (uses `p5,p50,p95`)  
```sh
curl "http://localhost:5173/api/v1/join/risk.user_transactions.txn_join/column/txn_by_user_transaction_amount_count_1h/summary?startTs=1672531200000&endTs=1677628800000"
```

#### Equivalent Explicit Default  
```sh
curl "http://localhost:5173/api/v1/join/risk.user_transactions.txn_join/column/txn_by_user_transaction_amount_count_1h/summary?startTs=1672531200000&endTs=1677628800000&percentiles=p5,p50,p95"
```

#### Custom Percentiles (`p0,p10,p90`)  
```sh
curl "http://localhost:5173/api/v1/join/risk.user_transactions.txn_join/column/txn_by_user_transaction_amount_count_1h/summary?startTs=1672531200000&endTs=1677628800000&percentiles=p0,p10,p90"
```

### Notes  
- Omitting the `percentiles` parameter is the same as explicitly setting
`percentiles=p5,p50,p95`.
- You can test using `curl` or Postman.  
- We need to let users change these percentiles via checkboxes or
another UI control.

## Checklist
- [ ] Added Unit Tests
- [ ] Covered by existing CI
- [ ] Integration tested
- [ ] Documentation update



<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Added support for customizable percentile parameters in summary data
requests, with a default setting of "p5, p50, p95".
- Enhanced the ability to retrieve detailed statistical summaries by
allowing users to specify percentile values when querying data.
  - Introduced two new optional dependencies for improved functionality.

- **Bug Fixes**
- Adjusted method signatures to ensure compatibility with the new
percentile parameters in various components.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: ken-zlai

api/src/main/scala/ai/chronon/api/Constants.scala

@@ -95,4 +95,6 @@ object Constants {
 
   // List entity type
   val ListEntityType: String = "entity_type"
+
+  val DefaultPercentiles: Seq[String] = Seq("p5", "p50", "p95")
 }

api/thrift/observability.thrift

@@ -158,5 +158,6 @@ struct JoinSummaryRequest {
     1: required string name
     2: required i64 startTs
     3: required i64 endTs
+    5: optional string percentiles  // Format: "p5,p50,p95"
     8: required string columnName
 }