plugins: extend API to allow file install

[teknoraver]

Extend the plugin API with two new methods:

• plugin_fsm_file_install_func
• plugin_fsm_file_archive_reader_func

The first one allows a plugin to take care of the file install, the second instead allows a plugin to handle the archive read.

This is used in the "reflink" plugin which uses CoW to extract archives

[pmatilai]

I've said this before, and I'm not going to repeat it all here: while I'm open to having these type of hooks added, RPMRC_PLUGIN_CONTENTS is not going to fly, this needs a different kind of API. See the discussion in and around #2416. The other conclusion was that to go forward, this will likely need a user that is not rpmextents, simply as a sanity reference. We're not interested in adding a hook that is only useful for one extremely specific use-case.

With the plugin API public now, we have an extra complication to deal with: the way things are, any change to that struct requires a soname bump. We will NOT bump the soname for 6.0 to lower the bar to entry, so the earliest opportunity for that is in 6.1.

[teknoraver]

@pmatilai I went through the comments.
Let's say I kill rpm2extents, and replace with another tool which reads the package from stdin, creates a directory like /var/cache/rpm/objects, and extracts every file with its hash as name.
Then, save the metadata somewhere in a sort of db, and the install will be just reflink (or copy) from the objects directory to the final path.

That will be acceptable as a solution?