[YSQL] Cost Based Query Optimization Improvements

[tanujnay112]

Jira Link: DB-1278
This issue tracks the effort to improve cost model and bring in basic functionality for more accurately differentiating between indexes.

Status	Feature	GitHub Issue
⬜️	Selectivity Estimation using pg_stats
⬜️	Index correlation estimation using pg_stats
⬜️	Select better index for a simple query with two indexes	#10840
⬜️	Update selfuncs.c::get_actual_variable_range() to support LSM trees	#11068
⬜️	Primary Key Selectivity is inaccurate	#12941
⬜️	Handle yb_hash_code clause selectivity	#11193
⬜️	Revisit costing functions for YB scans	#9494
⬜️	Define new cost model for joins	#12079
⬜️	I/O and CPU Costing
⬜️	Query planner sometimes estimates inordinately high costs for Index Scan	#6946
⬜️	Use geo-location (tablespace) as a cost component in query planner	#7535