Add the files from AUR repository to contrib/aur

Summary:
This is a first step toward automating the AUR packages deployment.
There are acually 2 git repositories (bitcoin-abc and bitcoin-abc-qt)
containing:

• A few utility files which are common to both packages. They are

located under contrib/aur/common.

• A PKGBUILD file which is a script that describes how to build the

package. It is different for each packages and located under
contrib/aur/<package>/PKGBUILD

• A .SRCINFO file which is regenerated at each update, and so is not

included in our tree.

For now these files do nothing, automation tools are coming in follow-up
diffs.

The AUR pages:

• bitcoin-abc: https://aur.archlinux.org/packages/bitcoin-abc/
• bitcoin-abc-qt: https://aur.archlinux.org/packages/bitcoin-abc-qt/

Test Plan: Clone from upstream and check there is no missing file.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7260