Make repo driver agnostics

[jubeless]

The repo is actually driver agnostic, thanks to Semiotics team for providing it. The task here is about removing "postgres" from the sink and everywhere. Here the smaller tasks to be performed here:

[!NOTE] Branch feature/deployable-unit should be used as a starting point, unless it has been merged already in which case develop should be used.

• Rename all Golang packages to be github.com/streamingfast/substreams-sink-postgres to become github.com/streamingfast/substreams-sink-sql
• Re-write README.md so that repository is clearly defined as an SQL agnostic sinker (I already started a bit I think)
• Rename Github repository to substreams-sink-sql
• Update documentation for all mentions of substreams-sink-postgres (we should probably check a few of our substreams-... repositories)
• Update the changelog to correctly show everything that needs to be changed with the rename and talk about the fact that is now driver agnostic (I already started a bit I think in CHANGELOG)
• Ensure substreams-sink-sql Substreams manifest is packaged now with every release (sfreleaser is able to package + attach)
• Perform the release

[sduchesneau]

All done, in a branch, gotta sync the tutorials and substreams documentation...

[maoueh]

We are just missing the release now, closing as no need to track it.