Implement makeJoinPlanCacheKey(const JoinGraph&) that serializes a normalized, parameterized join graph to an opaque string.

Pseudocode:

• Parameterize MatchExpressions
• Serialize nodes in ascending nodeId order: for each node emit nodeId:nss:parameterized_match_expr_shape
• Serialize edges in ascending edgeId order: for each each emit leftId:rightId:leftPath:rightPath:joinPredOperator

This can be unit tested independently.