Idea: Incorporate ClickHouse ordering into query planner?

Currently, `get_useful_pathkeys_for_relation()` only examines the list of expressions in each `EquivalenceClass` `EquivalenceClass` for each desired pathkey to see if there is an expression supported by ClickHouse. If there are none, or if any cannot be pushed down, it doesn't create a `PathKey`.

If, however, the relation has only a single ClickHouse table it ought to be possible to report the underlying sort order of the ClickHouse table, so the planner can take it into consideration when planning a query. Doing so would likely require annotating foreign tables with the underlying sort order.

IOW, while a sequential scan in Postgres should have zero `PathKey`s, a sequential scan of a ClickHouse table should have one for each column by which the table id ordered (or clustered) — or, rather 2 for each column, one ASC and one DESC.

Read get_useful_pathkeys_for_relation() and the handling of EquivalenceClass and PathKey first. Done means a single-table ClickHouse scan exposes the underlying table ordering as ascending and descending pathkeys so the PostgreSQL planner can use it.

Written by the indexing model from the issue text.