fix: Add sha256 support

What does this MR do and why?

Closes #34 (closed)

Add SHA256 support to the zoekt indexer

How to set up and validate locally

1. stop the zoekt indexer for gdk

   gdk stop gitlab-zoekt-indexer-development

2. checkout this branch and run make gdk to start the indexer running this code
3. enable a namespace for Zoekt indexing
4. setup a new project in that namespace that uses sha256

1. verify the project is searchable and returns results from code search
2. push a commit to the project
3. verify the updated changes are searchable