autoindexer: Allow commands to be configured in the index container pre-indexing

Created by: efritz

We cannot run lsif-clang usefully for auto-indexing currently because we do not expose a way to add steps into the index container which may be required to run the indexer (specify os packages that the code needs to compile, etc). Most of this for the majority of indexers can be done with pre-indexing steps in another container, as it requires only a file asset to be generated in the workspace. This doesn't work with C/C++ which may require things in system directories, etc.