[PROPOSAL]: Index view support

[sezruby]

This is an Uber issue to track Index view support. Please note that there is no specific plan for this yet.

Describe the problem
Currently, Hyperspace only allows to create an index on a relation node.
To move forward, we could support Index view which means index creation on an arbitrary logical plan.

Things to consider

• represent the source plan in index log entry file & re-generate the plan for refresh
  • class level serialization/deserialization is not compatible across different Spark versions.
  • SQL string can be an option
• Hybrid scan support
• code cleanup

Related issues

• Add signature computation support for arbitrary logical plans Add signature computation support for arbitrary logical plans #76 (closed)
• SparkSQL based index creation SparkSQL based index creation  #119
• Revisit logical plan serialization Revisit logical plan serialization #95

Describe your proposed solution
TBD

Describe alternatives you've considered
TBD

Additional context
TBD