make mariadb-plugin great

there is a mariadb-plugin tool. it's totally useless (MySQL introduced it and they later removed it, we didn't) so we want to give it completely new functionality. It'll be kind of like simple package manager for plugins. Usage examples:

$ mariadb-plugin search <plugin_name>

$ mariadb-plugin install <plugin_name>

$ mariadb-plugin uninstall <plugin_name>

The tool should

1. see how it itself was installed.
2. if it was installed from rpm, it executed dnf install for the plugin
3. ditto for apt and zypper.
4. otherwise (binary tarball) it downloads .tar.gz for the plugin and unpacks it into the basedir
5. depending on whether MDEV-40785 is done first, it should respect dependencies

Plugin download site(or mirror) should have plugin list with metadata for the binary tarball search command to work and the list of plugin files must be stored somewhere for binary tarball uninstall command to work.

This is the minimal step 1 functionality. Further commands (not part of this MDEV, just outlining possible directions):

• install <url> and install <path
• download — without installing, e.g. for later air-gapped install
• source — download source
• build
• dockerfile and compose
• addrepo and delrepo
  etc, etc