Update Snapshot and Dashboard Structs to more closely model the Grafana API

[git-ival]

• Add snapshots structs
• Update json deps to go-json-experiment/json
• Fix tests
• Update client

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[julienduchesne]

👋 I am closing this PR because this repository is now deprecated and will be archived. It will still be accessible in read-only mode.

We are now generating the Grafana client from its OpenAPI spec. Check out https://github.com/grafana/grafana-openapi-client-go for the new client. Consider opening an issue or a PR there if functionality is missing.