Refactor the code that "discover" which SAI components files an SSTable
has to allow making that code customizable. The default behavior remains
the existing one, that is the TOC file is consulted, with a fallback to
scanning disk if the TOC file is missing or corrupted.

The way to customize this behavior is similar to what `SSTableWatcher`
does: a new system property, `-Dcassandra.sai.custom_components_discovery_class`
allows to load a specific class (that extends the `IndexCoomponentDiscovery` class)
instead of the default.

As part of the refactor to make this new customisability easier, this
introduces a new `ComponentsBuildId` class that is the pair of the
version and generation of a group of components, since this is what
ultimately defines a given component "group" build.