[RFC] Support computed values.

For https://github.com/SatelliteQE/robottelo/pull/7774/

It is needed to allow some values to be computed at runtime, dynaconf already support lazy values using @jinja and @format but in this case it will be able to use any callable

Some fields can be computed at runtime and we can provide different API style to define it.

Options listed in comment bellow:

[rochacbruno]

Option 4: USe validators to provide default and computation

settings = Dynaconf(
    ...,
    validators=[
        Validator("KEY", lazy=callable, default="foo")
    ]
)

[rochacbruno]

Option 1: explicit dictionary mapping key to callable

from dynaconf import Dynaconf

settings = Dynaconf(
     computed={
        "name": callable_name
     }
)