port to bugzilla REST API

Summary:
https://bugzilla.readthedocs.io/en/5.0/api/

the old API is deprecated. the new API is RESTy and much easier to use
plus also has some efficiency boosts.

this commit introduces a new supporting library to wrap the REST API
for easy consumption in drkonqi.

- new internal lib wrapping the REST API
- unit testing of the lib
- xmlrpc use gone
- BugzillaManager continues to be a singleton manager for the API
- ancient cookie system for login is entirely gone, this has been
  deprecated (or perhaps even removed?) for years
- duplicate querying is pretty inefficient, needs porting to
  pagination, but is not worse than what it was before. in fact,
  it's faster than before out of the box!

the lib api is split in three pieces:

1. the connection containing protocol logic
2. the clients containing routing logic
2.1. command models that are basically POSIX-style argument structs which can serialize into json
3. the models deserializing the json API replies back into cpp

clients get a request, give back a job and the intended use is for
the consumer to then connect to the finished signal of the job to then
ask the clients for the final return value of the job.
this is a bit 'meh'. what I really wanted was a Future to be returned
and the caller to connect to the future. unfortunately qfuture is fairly
limited and since qtbase has no Promise implementation I'm rather opting
for the double-function-approach than doing it like we did in the previous
decade and have a gazillion signals.
this way the handling of the job is always uniform and all specific
"handling code" is inside the connected slot rather than the actual
connection.

error handling has been put entirely into exceptions which get raised
when trying to access a finished job. in BugzillaManager they get rescued
and turned into the already existing signals. this simplifies error
handling a fair amount and also makes the error handling code
look uniform across the various API calls we have.
the uniform representation of errors on the REST side thus always has
uniform representation on the c++ side.

Reviewers: apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19612