User identity authentication support

[kostrse]

This PR extends functionality of Azure token provider to support user identity authentication based on introduced earlier azcredentials.AadCurrentUserCredentials and user identity settings in azsettings.AzureSettings.

Original token_provider.go was refactored and token retrievers were moved into separated files retriever_msi.go, retriever_clientsecret.go. Implementation of aztokenprovider.AzureTokenProvider was split into two serviceTokenProvider and userTokenProvider, former is for user-agnostic service identities and the later is for user identities. They have different implementation of GetAccessToken func.

NewAzureAccessTokenProvider creates a new instance of serviceTokenProvider or userTokenProvider depending on type of credentials provided.

For user identity authentication to work:

• it needs to be configured in Grafana config (see Azure: Configuration for user identity authentication in datasources (Experimental) grafana#50277)
• it needs to be supported and allowed by the datasource.

Example of Azure configuration in datasource code (here):

authOpts := azhttpclient.NewAuthOptions(azureSettings)

// Allow user identity authentication for this datasource if it's configured in Grafana config
authOpts.AllowUserIdentity()

azhttpclient.AddAzureAuthentication(&clientOpts, authOpts, credentials)

httpClient, err := httpclient.NewProvider().New(clientOpts)

Fixes #6

[aangelisc]

Nice work @kostrse, this looks good and I've built and tested this locally and everything seems to work as expected 😊