recipetool: allow recipe create handlers to specify bitbake tasks to run

recipetool: allow recipe create handlers to specify bitbake tasks to run

When creating a recipe there can be cases where there is a class that
does some of the recipe creation (such as cargo-update-recipe-crates).

To avoid duplication of code, look for run_task assignments in the
extravalues dictionary returned by the handler, and if it is set then
call that task after writing the recipe.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>