Allow specifying ORDER BY in PostgreSQL cache policies

[apolukhin]

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See https://userver.tech/d9/dde/pg_cache.html#pg_cc_cache_policy

We need a way to say

  // `Order by` clause of the query.
  //
  // Required: no
  static constexpr const char* kOrderBy = "id DESC, updated ASC";

The ORDER BY functionality is useful when we have a table that stores all the events with some updated timestamp, but in cache we wish to store only the last event. To do that we need a guarantee on order of insert_or_assign calls

Tests also required

Internal ticket: 3248