Make query arguments required by default; add syntax to make them optional

[1st1]

Currently query arguments are optional but it makes more sense to make them required by default. E.g. in

SELECT <OPTIONAL int64>$0 + <int64>$1;

the $0 parameter would be optional and $1 would be required.

See also #1338 (comment)

In the context of this change we should also consider allowing use of SET OF in the argument cast, e.g.

SELECT sum(<OPTIONAL SET OF int64>$0);
# or
SELECT sum(<SET OF int64>$0);
# or
SELECT sum(<REQUIRED SET OF int64>$0);

Essentially, this all maps to the following cardinality-enforcement boundaries:

• <int64>$arg: cardinality is 1
• <REQUIRED int64>$arg: cardinality is 1
• <OPTIONAL int64>$arg: cardinality is at most 1
• <SET OF int64>$arg: cardinality is at least 0
• <REQUIRED SET OF int64>$arg: cardinality is at least 1
• <OPTIONAL SET OF int64>$arg: using OPTIONAL here doesn't make a lot of sense so we can prohibit this combination

[1st1]

@tailhook Do we need to update the docs? Could you also clarify what specifically was implemented in the merged PR? (we'll need that in changelog, cc @vpetrovykh)

[tailhook]

Documentation is in #1373. Shortly: by default arguments made required instead of optional and added <OPTIONAL type>$x; <REQUIRED type>$x is also supported (but no-op).

[elprans]

Done.