Add purge job for bicep artifacts

[sk593]

Description

This adds a purge job for test bicep artifacts. It will delete any manifests that are older than 90 days. This only applies to the test schemas uploaded as part of PRs, not any major release versions of Radius or AWS bicep types.

A successful test run can be found here: https://github.com/radius-project/radius/actions/runs/12939411131

Type of change

• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Radius (issue link optional).

Fixes: #issue_number

Contributor checklist

Please verify that the PR meets the following requirements, where applicable:

• An overview of proposed schema changes is included in a linked GitHub issue.
• A design document PR is created in the design-notes repository, if new APIs are being introduced.
• If applicable, design document has been reviewed and approved by Radius maintainers/approvers.
• A PR for the samples repository is created, if existing samples are affected by the changes in this PR.
• A PR for the documentation repository is created, if the changes in this PR affect the documentation or any user facing updates are made.
• A PR for the recipes repository is created, if existing recipes are affected by the changes in this PR.

[github-actions]

Unit Tests

3 552 tests  ±0   3 548 ✅ ±0   5m 30s ⏱️ +13s
  292 suites ±0       4 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 347c4a1. ± Comparison against base commit 7797bb0.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.83%. Comparing base (7797bb0) to head (347c4a1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8281      +/-   ##
==========================================
+ Coverage   59.81%   59.83%   +0.01%     
==========================================
  Files         590      590              
  Lines       39513    39513              
==========================================
+ Hits        23636    23642       +6     
+ Misses      14116    14112       -4     
+ Partials     1761     1759       -2     

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[ytimocin]

Do we need separate accounts for different operations? The other purge action uses AZURE_SP_TESTS_APPID. Wouldn't having one shared be better for actions that can be grouped together?