Add Case / When conditional function - #1421

piccolo.query.functions.conditional has Coalesce and NullIf, but there's no
way to express a SQL CASE statement. This adds Case and When, in the same
style as the existing functions.

from piccolo.query.functions import Case, When

await Band.select(
    Band.name,
    Case(
        When(Band.popularity > 900, then='super popular'),
        When(Band.popularity > 500, then='popular'),
        default='not popular',
        alias='popularity_label',
    ),
)

Because it's a QueryString, it also works in where clauses and order_by:

await Band.select(Band.name).order_by(
    Case(
        When(Band.name == 'Rustaceans', then=1),
        default=2,
    )
)

The one non-obvious bit: THEN values aren't always query parameters

Postgres can't infer the type of a bare parameter inside a CASE, so it assumes
text. If you bind an integer, asyncpg then rejects it before the query is even
sent:

CASE WHEN "band"."popularity" > $1 THEN $2 ELSE $3 END
asyncpg.exceptions.DataError: invalid input for query argument $2: 1 (expected str, got int)

Strings happen to work, which makes it an easy trap to fall into — the obvious
example works and the second one doesn't.

So get_case_value_string writes numbers, booleans and None into the SQL
directly, and parameterises everything else. There's no injection risk, as the
branch is only taken once the value is known to be a Python int / float /
Decimal / bool (non-finite floats and decimals fall back to a parameter).

Alternatives I considered, in case you'd prefer one of them:

• Wrap the values in Cast automatically. Needs a Python-type → SQL-type
  mapping which Piccolo doesn't have today, and the guesses are lossy (is an
  int an INTEGER or a BIGINT?). It's also risky on SQLite, where
  CAST(? AS TIMESTAMP) would mangle a datetime.
• Leave it to the user. Cast's own docstring already documents this class
  of problem, so there's precedent — but it makes the most obvious Case
  example fail, which felt like the wrong default.

Happy to change it either way.

Notes

• default is optional. Omitting it omits the ELSE, which is the same thing
  as ELSE NULL in SQL.
• alias defaults to "case", matching how Function derives its default
  alias — without it Postgres and SQLite name the column differently.
• When accepts a where clause (Where / WhereRaw / And / Or) or a
  QueryString, and then / default accept a column, another function, or a
  plain value.