Integrate postgres code

[Victor-N-Suadicani]

Purpose

The postgres feature has been removed from the SDK, so this crate (which I suppose is the only user of that feature) needs to integrate this functionality into itself, rather than relying on the SDK for it.

Changes

The postgres feature from the SDK was yanked out in Concordium/concordium-rust-sdk#204 - this PR adds this functionality back into this crate. The examples are brought over as well. The code is otherwise unchanged.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[soerenbf]

Looks good.

I don't see any reason why we'd need to update any of the code to conform to p7 other than updating the rust sdk to the correct version, as we don't depend on any of the API changes, and all the block item summaries (which could potentially include new baking/delegation events) are stored as JSONB