feat: show buckets estimation in explain

- Add new line in explain reporting on which buckets query will
  be executed.
- For queries consisting of a single subtree we can say exactly on
  which buckets it will be executed, for queries with more subtrees
  (with motions), we provide an upper bound of total buckets used
  in the query. Upper bound is computed by merging buckets from the
  leaf subtrees.
- In case for DML query with non-local motion we can't provide an
  upper bound, and print 'buckets: unknown'

Examples:
```
explain select a from t
->
projection ("t"."a"::integer -> "a")
    scan "t"
execution options:
    vdbe_max_steps = 45000
    vtable_max_rows = 5000
buckets = [1-3000]

explain select t.a from t join t as t2
on t.a = t2.b
->
projection ("t"."a"::integer -> "a")
  ...
execution options:
    vdbe_max_steps = 45000
    vtable_max_rows = 5000
buckets <= [1-3000]

explain select id from _pico_table
->
projection ("_pico_table"."id"::unsigned -> "id")
    scan "_pico_table"
execution options:
    vdbe_max_steps = 45000
    vtable_max_rows = 5000
buckets = any

explain insert into t values (1, 2)
->
insert "t" on conflict: fail
motion [policy: segment([ref("COLUMN_1")])]
    values
        value row (...)
execution options:
    vdbe_max_steps = 45000
    vtable_max_rows = 5000
buckets = unknown
```