Add resources context manager to set CPUs/GPUs for task

[ahuang11]

This PR allows users to specify required resources, like CPUs and GPUs, for a task.

To accomplish this, we collect resources kwargs from a user through a context manager (see example below), and then pass this to ray.remote internally in submit as suggested by Michael..

Closes #44

Example

Use 4 CPUs and 2 GPUs for the process task:

from prefect import flow, task
from prefect_ray.task_runners import RayTaskRunner
from prefect_ray.context import resources
@task
def process(x):
    return x + 1
@flow(task_runner=RayTaskRunner())
def my_flow():
    # equivalent to setting @ray.remote(num_cpus=4, num_gpus=2)
    with resources(num_cpus=4, num_gpus=2):
        process.submit(42)

Checklist

• This pull request references any related issue by including "Closes #<ISSUE_NUMBER>"
  • If no issue exists and your change is not a small fix, please [create an issue](https://github.com/{{ cookiecutter.github_organization }}/{{ cookiecutter.collection_name }}/issues/new/choose) first.
• This pull request includes tests or only affects documentation.
• Summarized PR's changes in [CHANGELOG.md](https://github.com/{{ cookiecutter.github_organization }}/{{ cookiecutter.collection_name }}/blob/main/CHANGELOG.md)

[desertaxle]

LGTM!