feat(Custom Targets): restrict Custom Targets domains

[aali309]

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Fixes: #323

Description of the change:

This change allows an environment variable to be configured so that...

Motivation for the change:

mentioned by: @andrewazores https://github.com/cryostatio/cryostat3/issues/323

How to manually test:

1. Run CRYOSTAT_IMAGE=quay.io... bash smoketest.bash...
2. ...

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No OpenAPI schema changes detected.

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CI build and push: All tests pass ✅ (JDK17) https://github.com/cryostatio/cryostat3/actions/runs/9290556247