schemadiff: EntityDiff has a 'Entities() (from Entity, to Entity)' function

[shlomi-noach]

Description

Entities() (from Entity, to Entity) return from "from" and "to" entities for the diff.

• When the diff is e.g. a AlterTableEntityDiff, this returns both the "from"'s CreateTable and the "to"s CreateTable.
• if the diff is a CreateTableEntityDiff then there's only a "to" ; "from" is nil.
• if the diff is a DropTableEntityDiff then there's only a "from" ; "to" is nil.

this is symmetrical in View diffs.

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

cc @dbussink

[shlomi-noach]

Cluster (20) is the remaining failing test. At this time we know it ia flaky and not rewuired. Merging.

[shlomi-noach]

Tracking issue: #10203