RP executor

[andre-merzky]

This PR implements a minimal RP executor. It accepts task requests and proxies them to the pilot instance created upon startup. The tasks are the run by the pilot and the executor gets a state notification upon completion.

Missing: the state notification is not yet communicated back to airflow, the dag thus stalls.

[andre-merzky]

Result collection now works.

Note that I had to execute the following steps in order to get the examples running:

$ pip install MDAnalysis
$ pip install alchemlyb
$ pip uninstall typing

The last triggered some module incompatibility within RP, I'l try to track this down (see radical-cybertools/radical.pilot/issues/3139).

I also needed to change the following values in airflow.cfg from

enable_xcom_deserialize_support = False
allowed_deserialization_classes = airflow\.*

to

enable_xcom_deserialize_support = True
allowed_deserialization_classes = airflow.*

For the latter, see apache/airflow#34093 - the fix was merged but is not yet released.

In order to use the executor, please create a file like this:

$ cat ~/.radical/pilot/configs/resource_local.json
{
    "localhost": {
        "rp_version"    : "installed",
        "virtenv_mode"  : "local"
    }
}