Add Pod.gen()

[jacobtomlinson]

With kubectl you can call commands like kubectl run nginx --image=nginx which will generate the spec for a Pod and create it. Or you can generate a service with kubectl create service clusterip my-cs --tcp=5678:8080.

This PR adds similar functionality with a .gen() method on Pod which allows you to generate the spec of a Pod object with a few keyword arguments. We can easily extend this for other objects in the future.

from kr8s.objects import Pod

pod = Pod.gen(name="example-1", image="nginx:latest")
pod.create()

xref #142

[codecov]

Codecov Report

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

Comparison is base (0280954) 94.99% compared to head (1410d55) 94.87%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
kr8s/portforward.py	57.14%	3 Missing ⚠️
kr8s/tests/test_gen.py	90.47%	2 Missing ⚠️
kr8s/_objects.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #222      +/-   ##
==========================================
- Coverage   94.99%   94.87%   -0.13%     
==========================================
  Files          26       27       +1     
  Lines        2579     2633      +54     
==========================================
+ Hits         2450     2498      +48     
- Misses        129      135       +6     

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