Enhancement: Implement tar source plugin

From https://wiki.gnome.org/Projects/BuildStream/Roadmap/SourcePlugins

Tarball Source This should simply use host wget to obtain and cache tarballs in the local users source directory.

The source should make a sha256 sum of the downloaded tarball in the track() method, so that users need not list the checksums themselves.

The source should be able to detect and extract the tarball by itself, regardless of compression types, using host tar.

If it is not good enough to derive what compression format from the file extension, then a configuration option for the source can be added for the tarball Source so a user can explicitly say this tarball is bzip2.

A source plugin needs to implement:

• configure
• preflight
• get_unique_key
• get_consistency
• get_ref
• set_ref
• track
• fetch
• stage