[2pt] new option "exclude_null=true/false"

[TarantoolBot]

Added new option for index part definition,
that allows index to skip tuples with null at this part.
By default the option is turned off.
When turned on, "is_nullable=true" option will be set automatically.
It can't be turned on for the primary key.
Option can be changed dynamically (in this case index is rebuilt).
Such index does not store filtered tuples at all,
so indexing can be done faster.
SQL: such index is skipped in select statements
unless explicitly specified.

exclude_null and is_nullable are connected, so
this table describes the result of combining them.

Exclude NULL/Nullable	False	True
False	+	+
True	- *	+ **

* is not allowed
** Partial index
Requested by @mary3000 in tarantool/tarantool@17c9c03.

[Onvember]

SQL: such index is skipped in select statements unless explicitly specified.

@pgulutzan we have here a note for SQL. Where should we write it on our doc?

[pgulutzan]

@Onvember:

https://www.tarantool.io/en/doc/latest/reference/reference_sql/sql_statements_and_clauses/#indexed-by-clause
i.e. ./reference/reference_sql/sql_statements_and_clauses.rst
after the words "Ordinarily Tarantool chooses the appropriate index or lookup method depending on a complex set of “optimizer” rules; the INDEXED BY clause overrides the optimizer choice."
The new feature could be ignored with the reasoning that we never claim what index will be chosen anyway, but I think it would be okay to add
"If the index was defined with the is_nullable parts option, it will only be used if the user specifies it."

Incidentally I notice that according to tarantool/tarantool#4480 this is milestone 2.8.2.