Enable specification of target hostname for a dragon task (#660)

## Description

This PR adds two features:

1. Ability to specify hostnames that tasks should run on
2. Enable tasks colocation

### Specifying Hostnames

The existing `DragonRunRequest` supported the ability to specify a
hostname when creating a policy used to run a task. However, the
hostnames were not exposed to clients.

This ticket allows clients to pass a list of hosts that will be used in
place of the default "first available host" behavior.

### Task Colocation

The prior system for finding nodes to execute a task worked worked only
with unassigned nodes. Any node assigned a task could not be assigned
another task.

This ticket adds a more capable prioritizer class that enables clients
using hostnames to colocate tasks. It also retains the capability to
return open nodes when no hostname is specified.

doc/changelog.md

@@ -13,6 +13,7 @@ Jump to:
 
 Description
 
+- Enable hostname selection for dragon tasks
 - Remove pydantic dependency from MLI code
 - Update MLI environment variables using new naming convention
 - Reduce a copy by using torch.from_numpy instead of torch.tensor

smartsim/_core/launcher/dragon/dragonLauncher.py

@@ -170,6 +170,7 @@ def run(self, step: Step) -> t.Optional[str]:
             merged_env = self._connector.merge_persisted_env(os.environ.copy())
             nodes = int(run_args.get("nodes", None) or 1)
             tasks_per_node = int(run_args.get("tasks-per-node", None) or 1)
+            hosts = str(run_args.get("host-list", ""))
 
             policy = DragonRunPolicy.from_run_args(run_args)
 
@@ -187,6 +188,7 @@ def run(self, step: Step) -> t.Optional[str]:
                         output_file=out,
                         error_file=err,
                         policy=policy,
+                        hostlist=hosts,
                     )
                 ),
                 DragonRunResponse,