Add `available` property to cert_handler #158

Job	Run time
PR / Check against ignorelist	4s
PR / Quality Checks / Check the CHARMHUB_TOKEN secret	1s
PR / Quality Checks / Linting / Lint	21s
PR / Quality Checks / Scenario Tests / Scenario tests	12s
PR / Quality Checks / Static Analysis / Static Analysis of Charm	15s
PR / Quality Checks / Inclusive naming / check	10s
PR / Quality Checks / Static Analysis / Static Analysis of Libs	19s
PR / Quality Checks / Unit Tests / Unit tests	14s
PR / Quality Checks / Check libraries	43s
PR / Quality Checks / Integration Tests / Integration Tests	15m 53s
PR / Quality Checks / CodeQL analysis / Analyze (python)	2m 13s
	20m 25s

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