Maybe paren for expr inside fragment

[dkuku]

Problem: Boolean operations (and, or) inside fragment expressions were not being
wrapped in parentheses, causing SQL precedence issues. For example:
fragment("?::integer", r.x and r.y) generated s0."x" AND s0."y"::integer
But should be (s0."x" AND s0."y")::integer

closes #4688

Test output before the implementation, now the test are passing.

  1) test fragments (Ecto.Adapters.TdsTest)
     test/ecto/adapters/tds_test.exs:690
     Assertion with == failed
     code:  assert all(query) == ~s{SELECT CAST((s0.[x] AND s0.[y]) AS INT) FROM [schema] AS s0}
     left:  "SELECT CAST(s0.[x] AND s0.[y] AS INT) FROM [schema] AS s0"
     right: "SELECT CAST((s0.[x] AND s0.[y]) AS INT) FROM [schema] AS s0"
     stacktrace:
       test/ecto/adapters/tds_test.exs:724: (test)

...............................................................................................................................................................................................

  2) test fragments (Ecto.Adapters.MyXQLTest)
     test/ecto/adapters/myxql_test.exs:645
     Assertion with == failed
     code:  assert all(query) == ~s{SELECT CAST((s0.`x` AND s0.`y`) AS INT) FROM `schema` AS s0}
     left:  "SELECT CAST(s0.`x` AND s0.`y` AS INT) FROM `schema` AS s0"
     right: "SELECT CAST((s0.`x` AND s0.`y`) AS INT) FROM `schema` AS s0"
     stacktrace:
       test/ecto/adapters/myxql_test.exs:686: (test)

....................................

  3) test fragments (Ecto.Adapters.PostgresTest)
     test/ecto/adapters/postgres_test.exs:828
     Assertion with == failed
     code:  assert all(query) == ~s{SELECT (s0."x" AND s0."y")::integer FROM "schema" AS s0}
     left:  "SELECT s0.\"x\" AND s0.\"y\"::integer FROM \"schema\" AS s0"
     right: "SELECT (s0.\"x\" AND s0.\"y\")::integer FROM \"schema\" AS s0"
     stacktrace:
       test/ecto/adapters/postgres_test.exs:868: (test)
       ```

[josevalim]

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