Update backend to common AzureCredentials

[kostrse]

Grafana Azure SDK provides hierarchy of domain entities that describe Azure credentials:

• azcredentials.AzureCredentials - the comon credentials interface
• azcredentials.AzureManagedIdentityCredentials - Managed Identity (MSI) credentials
• azcredentials.AzureClientSecretCredentials - App Registration (client secret) credentials

ADX has additional authentication method, On-Behalf-Of, which is not supported by Grafana Azure SDK and could be defined locally in the ADX plugin code:

// inherits azcredentials.AzureCredentials interface
type OnBehalfOfCredentials struct {

	AzureCloud   string
	Authority    string
	TenantId     string
	ClientId     string
	ClientSecret string
}

func (credentials *OnBehalfOfCredentials) AzureAuthType() string {
	return "clientsecret-obo"
}

Credentials should be parsed from JsonData during datasource initialization and passed over further as an abstract AzureCredentials object (see Azure Monitor example here).

NewServiceCredentials here doesn't need to receive raw models.DatasourceSettings as an input (at least not for credentials) but a AzureCredentials object.

The ADX plugin has been using own schema for storage of credentials in JsonData which requires custom parser. New implementation should first try to read credentials in the common format using standard parser azcredentials.FromDatasourceData and if credentials not found using this parser then fallback to a legacy parser and read them in legacy format.
This would allow to migrate ADX datasource into the new format of credentials in JsonData while preserving compatibility with existing format.