[R] Update as.Date() to support direct casting from double/float

[asfimport]

This might need support from C++ as, at present, it is not possible to cast directly from a double. Currently, we are flooring the double to an integer as an intermediate step.

Reporter: Dragoș Moldovan-Grünfeld / @dragosmg

Related issues:

• [R][C++] Discussion: Plans for date casting from int to support an origin option? (depends upon)

_{Note: This issue was originally created as ARROW-15807. Please see the migration documentation for further details.}