Expose Random() as a function instead of an expression.

[Nick Pope]

This follows on from #31839 where all other proposals were rejected, but exposing Random() as a function was accepted.

I also propose unifying the random number returned to be in the range [0.0, 1.0) to make it more useful as the backend support differs:

Backend	Function	Type	Range	Notes
PostgreSQL	​RANDOM	double precision	[0.0, 1.0)
MySQL	​RAND	double precision	[0.0, 1.0)
MariaDB	​RAND	double precision	[0.0, 1.0)
Oracle	​DBMS_RANDOM.RANDOM	integer	[-2^31, 2^31)	Currently used in Django, but Oracle have deprecated it.
Oracle	​DBMS_RANDOM.VALUE	number	[0.0, 1.0)
SQLite	​RANDOM	integer	[-2^63, 2^63)	We can use Python's random.random() for [0.0, 1.0)

This shouldn't adversely affect existing use cases of the expression as it was undocumented and used to support .order_by('?').

We should steer away from setting the random seed as it varies wildly between backends.
We should also avoid any backend-specific arguments for the upper and lower limits for the random number.

For anyone arriving here that wants to generate a random integer from a random number in the range [0.0, 1.0) see the MySQL documentation for a good example.

[Nick Pope]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 06c5d3f:

Fixed #32060 -- Added Random database function.