Add a "user" parameter to the Context Macro to control the current user

The idea would be to use this macro when you need to run some code using the passed user. For example:

{{impersonate user="SomeUser"}}
... at this point the current user in the context is SomeUser... 
{{/impersonate}}

Use cases:

• Imagine some server calling remotely (using HTTP) a wiki page inside an XWiki server, without any authentication. Now imagine that thus guest user need to access some other page that is view-protected (because it contains sensitive data).

Note 1: this macro would require PR.
Note 2: we could name it differently. Alternative proposals:

• {{executeAsUser}}
• {{evaluateAsUser}}
• {{contextUser}}
• {{currentUser}}

FTR I had to implement this and I did it like this, in velocity (I didn't put a #try since I wanted exception throwing but ideally we'd need a try/finally):

## WARNING: This page must be saved/installed by a user having Programming Rights!
## We use the author of this page as the current user to execute the code since the Licensing code is protected
## and only viewable to administrators for privacy reasons.
#set ($originalCurrentUserReference = $xcontext.context.userReference)
#set ($discard = $xcontext.context.setUserReference($doc.contentAuthor))
...
#set ($discard = $xcontext.context.setUserReference($originalCurrentUserReference))