Allow the request ID to be passed in via the `X-Request-Id` header

Allow the request ID to be passed in via the `X-Request-Id` header

For tracing and logging purposes, we want to be able to see/generate the
list of all of the requests that happen in the environment for a given
external incoming request. To that end, allow Mediawiki to accept the
request ID provided by the incoming request as its own.

Since this may be problematic for set-ups that don't have an entity in
front of MW that sanitises the headers on the way in, introduce a new
global variable, `$wgAllowExternalReqID`, that can disable this
behaviour. By default, the feature is disabled.

Bug: T201409
Change-Id: I605471fb8b5bbc290baeecc7d80d9d715cb240c9