Add budget to image and dataset data model

[Mosqidiot]

Follow up for: #306

Tested the newly released beaker-py, image creation and dataset creation looks good but there is a warning about the data model is outdated for both, this PR fixed the data model.

> uv run --with "beaker-py==v1.38.0" --with "docker" python -c "from beaker import Beaker; Beaker.from_env().image.create(name='test-image-2', image_tag='busybox:latest', budget='ai2/compute')"
6aba5e0d32d9: Layer Already Exists ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100%  
/Users/fangzhouh/.cache/uv/archive-v0/5_9V_H3zzXZYDSzqd0VNa/lib/python3.13/site-packages/beaker/util.py:25: RuntimeWarning: Found unknown field 'budget: 01JYHSJ426K2R2ZN6ZT45RMG97' for data model 'Image'. This may be a newly added field that hasn't been defined in beaker-py yet. Please submit an issue report about this here:
https://github.com/allenai/beaker-py/issues/new?assignees=&labels=bug&template=bug_report.yml
  warnings.warn(
>  uv run --with "beaker-py==1.38.0" --with "docker" python -c \
"from beaker import Beaker; Beaker.from_env().dataset.create('test-again', './diff_output_1.txt', budget='ai2/compute')"
/Users/fangzhouh/.cache/uv/archive-v0/5_9V_H3zzXZYDSzqd0VNa/lib/python3.13/site-packages/beaker/util.py:25: RuntimeWarning: Found unknown field 'budget: 01JYHSJ426K2R2ZN6ZT45RMG97' for data model 'Dataset'. This may be a newly added field that hasn't been defined in beaker-py yet. Please submit an issue report about this here:
https://github.com/allenai/beaker-py/issues/new?assignees=&labels=bug&template=bug_report.yml
  warnings.warn(
Uploading dataset ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:00 0 bytes/s 2.8/2.8 kB

[epwalsh]

LGTM!

[github-actions]

This PR has been released in v1.38.1.