Load pickup repository via configuration

[jamshale]

This pull request allows the mediator to load a pickup repository through environment variables and config values. It only currently allows the postgres pickup repository to be loaded but adding a loader for the dynamodb pickup should be straight forward.

[TimoGlastra]

I made similar changes in #55, but with a slightly different approach. I'm fine with merging this PR and I'll incorporate the changes.

I can then also do the update to zod-config to have strongly typed and validated config (i think if we allow multiple options for storage etc it will be good to be strict)

[esune]

I am not yet too familiar with this codebase so will wait for @TimoGlastra or @genaris to approve, but went over the changes and things look good to me.

[genaris]

Overall LGTM, but I think the setup can be a bit simplified to make easier to use

[genaris]

I think we can simplify this setup by just specifying the pickup type, and choosing automatically the strategy depending on it. For postgres I think QueueOnly is the only one that actually makes sense, while we can use QueueAndDeliver for the in memory implementation.

[jamshale]

I've been working with the postgres pickup quite a bit and have a fork that allows live mode by only closing the database sessions when a websocket session is closed. I've tested it quite a bit in a clustered environment and it seems like all is well and websockets successfully change instances. I was going to create a separate PR to allow both delivery modes.

I'm curious what your concerns are for live mode with the postgres pickup repository? If there's a problem I'm not seeing? I think there would be an issue with stale sessions that would need to addressed for live mode but I haven't seen any other major problems.

[esune]

I'd be good with merging as-is, changes look good and we can tweak in the future as/if needed