Add plan signature validation for Hybrid Scan

[sezruby]

Describe the issue

Currently, Hyperspace only supports creating indexes on a logical relation node. In order to support arbitrary logical plans, plan signature comparison is required for getting candidate indexes. This work is done by #76. In case of hybrid scan, we cannot utilize the "index" signature value in IndexLogEntry which is a composite of FileBasedSignature + PlanSignature, because

• Index signature calculation overhead during the query time
• Delete support - need to identify the deleted files and exclude them in the index signature calculation

#153 handles this issue by checking the metadata of source files directly without using the index signature. As a follow-up, we need to add plan signature validation for hybrid scan accordingly. This could be done by following:

1. Store the plan signature of the source plan separately in IndexLogEntry
2. Compare plan signatures in getCandidateIndex

To Reproduce

Expected behavior

Environment

[imback82]

@sezruby Can you check if this issue can be closed?

[sezruby]

Since we only allow 1 relation and the current plan signature of non-hybrid scan is just using "Relation" node, I think we don't need to consider the query plan(Relation) for Hybrid Scan - until we support #95.