feat: Add GetBudgetByID function for enterprise budgets

[wtneff]

This PR adds a function to match the getbudgetbyid endpoint in the API for Enterprise Budgets.

Resolves #4408

[google-cla]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (cd248c6) to head (7b36dea).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4563   +/-   ##
=======================================
  Coverage   98.57%   98.57%           
=======================================
  Files         197      197           
  Lines       18299    18308    +9     
=======================================
+ Hits        18038    18047    +9     
  Misses        261      261           

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[gmlewis]

After you resolve the CLA issue and the linter failures by running step 4 of CONTRIBUTING.md, we can move forward with this PR, @wtneff.

[wtneff]

Closing in favor of #4565, which has a clean committer/author history.