Add initial support for Pod.exec()

[jacobtomlinson]

This PR adds an initial implementation of Pod.exec() to enable running commands in remote containers.

from kr8s.objects import Pod

pod = Pod.get("my-pod")

command = pod.exec(["uptime"])
print(command.stdout.decode())
# 13:49:05 up 23:03,  0 users,  load average: 0.66, 0.87, 0.85

The Pod.exec command is intended to roughly mimic the behaviour of subprocess.run() and supports redirecting stdout/stderr from the container.

import sys
from kr8s.objects import Pod

pod = Pod.get("my-pod")

# Call `ls /` and direct stdout/stderr to the current process
# Also skip checking the return code of the command to avoid raising an exception
pod.exec(["ls", "/"], stdout=sys.stdout.buffer, stderr=sys.stderr.buffer, check=False)

Closes #209
Supersedes #192

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (dde21b5) 95.03% compared to head (61ad3ae) 95.06%.

Files	Patch %	Lines
kr8s/_exec.py	93.84%	4 Missing ⚠️
kr8s/tests/test_objects.py	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
+ Coverage   95.03%   95.06%   +0.03%     
==========================================
  Files          25       26       +1     
  Lines        2458     2576     +118     
==========================================
+ Hits         2336     2449     +113     
- Misses        122      127       +5     

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