Summary:
There is some code duplication for determining whether we can use query
optimizations due to the fact that the table/index being scanned are
colocated or copartitioned.  This code is duplicated in yb_scan.c and
ybginscan.c.  Deduplicate to a common function YbIsScanningCohostedRels.

In doing so, find the code is inconsistent, so create issue #25940 for
that and add a bunch of todos referencing it.  Also, rename
YbcPgPrepareParameters.querying_colocated_table to
YbcPgPrepareParameters.embedded_idx.

Jira: DB-15286

Test Plan:
jenkins

Close: #25959

Reviewers: tnayak, #db-approvers, smishra

Reviewed By: tnayak, #db-approvers, smishra

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D41743