Create new linker._supports_seed method

[ThomasHepworth]

An extension of #1161.

If a specific backend doesn't support random seeds, I think it makes more sense to exit at the start of a given method, rather than halfway through the execution.

For example, the sqlite linker will only halt execution when it reaches _random_sample_sql, but it should probably exit at the start of the u training step.

Yeah I think that would be useful - same applies to `sqlite`. Maybe need to end up with `linker._supports_seed` or something

Originally posted by @ADBond in #1251 (comment)