Add standalone MessagePickupRepository Postgres implementation

[genaris]

Since there are some interest in the community to use a simpler solution than this client-server architecture, I think it would be nice to add (with some upgrades) the previous approach we followed in 2060.io DIDComm Mediator about using directly.

This will be a new package, completely independent from client and server, that will be published in NPM, so it can be used by any mediator. We can put it under packages/postgres and publish it in NPM with the name @2060.io/credo-ts-message-pickup-repository-pg.

Of course, we will need to do a good refactoring there to make it generic for any kind of Agent and to remove some unneeded parameters.