Simplify the MAS config & allow overriding the Docker image

[sandhose]

The intent is so that we can use the same image update mechanism from the element-web repo, so that we regularly pull MAS 'main' and pin it.

I've also simplified the initial config to remove most of the 'default' fields

It also re-exports the Postgres testcontainer to make it easier to consume in EW

[t3chguy]

This is great, but removing so many config options means tests can't override them without having to force types, as we infer the type from the default config. Unless those options have truly gone away should we keep some, e.g. branding?, policy wasm module?

[sandhose]

@t3chguy Makes sense. We do have a JSON schema for the config, and json-schema-to-typescript seems to generate a decent output: https://gist.github.com/sandhose/98345404b28ac359fff8ba7801dacde7

I can do a one-shot generation for now, WDYT?

[t3chguy]

Sure!

[sandhose]

Have done that in e4d64aa