Rate limit sessions for throttling control

[kostrse]

When using service identity authentication in datasource, query requests originating from different users share the same identity, and may be seen my the resource endpoint as coming from the same user.

I may occasionally trigger throttling problems in large Grafana instances with popular dashboards.

This PR introduces an additional session header X-RateLimit-Session, different for each Grafana user, which provide extra information to the resource endpoint regarding the different user sessions which share the same service identity for authentication.

This session header needs to be enabled by the datasource:

authOpts := azhttpclient.NewAuthOptions(azureSettings)
authOpts.Scopes(scopes)

// Enable session header
authOpts.AddRateLimitSession(true)

azhttpclient.AddAzureAuthentication(clientOpts, authOpts, credentials)

To make sessions work, datasource should also include context of the signed-in user to the request context, as implemented in the ADX datasource here or here. This allows the Azure middleware to use the passed user's information to generate session ID.

Resource endpoint should know about the header and use it for its rate limit calculations.

[aangelisc]

One nit in the comments to be addressed before we merge this 😊