Add setUserCookie method on cookie manager

[xiao-lix]

To address this issue: ai_user cookie not present after reenabling the cookie #1585

Reason:
ai_session cookie is present because after reenabling the cookie, renew on session is called when property plugin processes next telemetry. But ai_user cookie only "created" as part of the creating (loading) of the SDK. Thus, for SPA applications, there won't be "reloading" and therefore "creating" an instance. So ai_user cookie is not present.

Solution:
Similar to SessionManager, add a update method on User and the method is triggered in propertiesPlugin processTelemetry. Once customer re-enable the cookie usage, the previous created userId is used to generate cookie which is set by cookie manager.

Sample Usage:

const ai = new ApplicationInsights({
    config: {
        instrumentationKey: '<YOUR_IKEY>',
        disableCookiesUsage: true // disable cookie usage on initialize
    }
});
ai.loadAppInsights();
ai.getCookieMgr().setEnabled(true); // re-enable cookie usage for both session cookie and user cookie

Sample result:
Before re-enable cookies, no ai_session or ai_user cookie created.
After re-enable cookies and set user cookie, ai_session and ai_usertest cookies are created:

[MSNev]

This issue is not about adding another method to the cookie manager (which we should not do), as it already has all of the necessary set methods for cookies required.

The suggestion I had in the issue was to add this method to the User object (same as the setAuthenticatedUser()) as the application just wants to "re-enable" cookies and as such trigger that any configured cookies can now be re-created.

So this would apply to both ai_user and ai_session, so it might mean that we put this on the telemetry context rather than the user -- etc.