Literal and field name redaction for ExpressionFieldPath and ExpressionConstant

As part of generating the telemetry key for an aggregation pipeline, we need to extend the serialization logic for all the aggregation expression types to support literal value redaction as well as field name redaction.

For most aggregation operators, we'll want to replace literal values with a generic placeholder. This will require updating the serialization method of ExpressionConstant.

We will also need to hash all user supplied field names, paths, and variables. This will require updating the serialization method of ExpressionFieldPath. Note that we will want to preserve MQL system variable references.