DataSourceSettings: add support for expandable details

[vickyyyyyyy]

What this PR does / why we need it:

In our enterprise plugins, in the ConfigEditor, we would like to be able to display more detailed error messages that are expandable. This would be helpful for stack traces or more technical errors (that can be collapsed by users).

Before

The alert component only looks at the message from the details payload:

{testingStatus.details?.message ?? null}

After

In the plugin's CheckHealth, we can additionally pass in a verbose error message in the details:

response, err := datasource.API.TestDatasource(ctx)
if err != nil {
    jsonDetails := `{"message":"Could not connect to AppDynamics. This usually happens when the URL is incorrect.", "verboseMessage":"More error details here when expanded."}`

    return &backend.CheckHealthResult{
        Status:  backend.HealthStatusError,
        Message: "Authentication error (401)",
        JSONDetails: []byte(jsonDetails),
    }, nil
}

And this will be picked up by the page:

Which issue(s) this PR fixes:

Fixes https://github.com/grafana/integrations-team/issues/146

Special notes for your reviewer:

TIA 🙏

[CLAassistant]

All committers have signed the CLA.

[ryantxu]

can we also add a go struct to the SDK with this same shape?

[vickyyyyyyy]

Do you mean replace the []byte type for JSONDetails https://github.com/grafana/grafana-plugin-sdk-go/blob/master/backend/diagnostics.go#L63 with something like this

type CheckHealthResultsJSONDetails struct {
	Message string `json:"message,omitempty"`
	VerboseMessage   string `json:"verboseMessage,omitempty"`
}

so we are able to define the specific fields we are expecting and be more strict with the keys or do you mean something else?