Add with_ clause for Common Table Expressions

[SahilRajputdotexe]

Closes #1439

This adds support for WITH clauses (Common Table Expressions) on Select, Update, Delete and Insert queries.

You build a CTE from a name and a body, which can be a query or a QueryString, and pass it to with_. CTE.recursive makes a WITH RECURSIVE expression from a base and a recursive part, and materialized=True/False adds the MATERIALIZED / NOT MATERIALIZED hint. CTE.ref() returns a CTERef for the declared column_names, so the columns can be referred to by name. Mistakes like an invalid name, a duplicate name or a column count that doesn't match raise CTEError.

reports = CTE.recursive(
    "reports",
    base=QueryString('SELECT "id", 0 FROM "employee" WHERE "id" = {}', 2),
    recursive=QueryString(
        'SELECT "employee"."id", "reports"."depth" + 1 FROM "employee" '
        'JOIN "reports" ON "employee"."manager_id" = "reports"."id"'
    ),
    column_names=["id", "depth"],
)

await Employee.select(Employee.name).where(
    Employee.id.is_in(QueryString('SELECT "id" FROM "reports"'))
).with_(reports)

The CTE is referred to from the main query through raw SQL like above. I didn't try to add joins against a CTE, to keep this small.

I added a docs page under query clauses, and tests/query/test_cte.py, which checks the generated SQL and runs select, update, delete and insert against the test database. isort, black, flake8, mypy and slotscheck are clean, and I ran the new tests with SQLite locally and the Postgres and CockroachDB jobs on my fork.