Allow access log to also access the exchange data

[pilhuhn]

Description

The (Vert.x) routing context has a generic data store. Allow to log entries from that store in access log.

In our project we have a need to e.g. also log the tenantId of a logged in user in addition to the login-name.
We put that tenant id into the routing context

routingContext.put(X_RH_ACCOUNT, rhIdentity.identity.accountNumber);

Now it would be neat to easily log that value
E.g.

2022-06-24 09:05:48,960 INFO [access_log] (executor-thread-2) "127.0.0.1 - joe-doe-user 24/Jun/2022:09:05:48 +0200 "GET / HTTP/1.1" 403 - accountId= 1234"

Implementation ideas

Follow the example of other AccessLog attribute handlers in vertx-http

[pilhuhn]

@gsmet Shall I just close this one?

[gsmet]

I adjusted the description of the PR using Fixes so that the issue is linked to the PR properly.