Implement `grafana_datasource_exchange` #978

Job	Run time
PR / Check against ignorelist	8s
PR / Quality Checks / Check the CHARMHUB_TOKEN secret	1s
PR / Quality Checks / Static Analysis / Static Analysis of Charm	40s
PR / Quality Checks / Linting / Lint	20s
PR / Quality Checks / Scenario Tests / Scenario tests	34s
PR / Quality Checks / Inclusive naming / check	6s
PR / Quality Checks / Static Analysis / Static Analysis of Libs	27s
PR / Quality Checks / Unit Tests / Unit tests	1m 9s
PR / Quality Checks / Check libraries	30s
PR / Quality Checks / CodeQL analysis / Analyze (python)	1m 21s
PR / Quality Checks / Integration Tests / Integration Tests	1h 25m 39s
	1h 30m 55s

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