API for pessimistic locks (`SELECT … FOR UPDATE`)

[crionuke]

Hi, there!
There is any plans to implement some locking api for spring r2dbc (may be with R2dbcEntityTemplate)
as it was in classic spring data jpa like this

@Lock(LockModeType.PESSIMISTIC_WRITE)
List<User> findByLastname(String lastname);

And of course, would be glad to see any examples of workaround to implement pessimistic locking used current version of spring-data-r2dbc

[DiegoKrupitza]

My current draft only considers derived queries so far, since this does not require tinkering around with the (native) custom query.
I hope I can create the PR for R2DBC soon.