refactor(pageserver): designated api error type for cancelled request (#7949)

Closes #7406.

## Problem

When a `get_lsn_by_timestamp` request is cancelled, an anyhow error is
exposed to handle that case, which verbosely logs the error. However, we
don't benefit from having the full backtrace provided by anyhow in this
case.

## Summary of changes

This PR introduces a new `ApiError` type to handle errors caused by
cancelled request more robustly.
-  A new enum variant `ApiError::Cancelled`
- Currently the cancelled request is mapped to status code 500.
- Need to handle this error in proxy's `http_util` as well.
- Added a failpoint test to simulate cancelled `get_lsn_by_timestamp`
request.

Signed-off-by: Yuchen Liang <yuchen@neon.tech>

proxy/src/serverless/http_util.rs

@@ -45,6 +45,10 @@ pub fn api_error_into_response(this: ApiError) -> Response<Full<Bytes>> {
             err.to_string(),
             StatusCode::REQUEST_TIMEOUT,
         ),
+        ApiError::Cancelled => HttpErrorBody::response_from_msg_and_status(
+            this.to_string(),
+            StatusCode::INTERNAL_SERVER_ERROR,
+        ),
         ApiError::InternalServerError(err) => HttpErrorBody::response_from_msg_and_status(
             err.to_string(),
             StatusCode::INTERNAL_SERVER_ERROR,