Create and support a PPA

As suggested by @aahlenst in https://github.com/AdoptOpenJDK/openjdk-installer/issues/10#issuecomment-606524942, I open a new issue, because I think many downstream PPA maintainers would benefit of AdoptOpenJDK being published in a PPA.

### Rationale

I want to use AdoptOpenJDK as a build time dependency in my own PPA. However, one can only depend on packages within Ubuntu's main repo or on packages from different PPAs.

In order to publish packages in a PPA, you need to create a debian source file (`debuild -S`) and upload it together with the original source taball to the build system. The build itself will run "offline", i.e. we can not download additional packages at build time. Therefore it is not possible to depend on @rpardini's PPA, which just downloads the pre-built binaries.

### Alternatives

One could use Ubuntu's OpenJDK builds, but they [have some issues preventing me from using `jlink`](https://bugs.launchpad.net/ubuntu/+source/openjdk-14/+bug/1868699). Also, one can't use recent OpenJDK builds on older distros and have to live with their release cycle.

Start by examining the installer's existing Debian packaging and Ubuntu build workflow; the issue does not name specific files or tests. Review how a source package can build offline and how a PPA would publish AdoptOpenJDK, then define done as a supported PPA that downstream PPA builds can depend on.

Written by the indexing model from the issue text.