Partkey REST API - Use messagepack format

As a transition step, the partkey REST API is using the old partkey file format. We should define and support a new file format that is a messagepack encoded object instead of the sqlite DB file.

The "new" format can be the msgpack formatted `ParticipationRecord` object.

For transition purposes, the REST API should support both formats.

## Dependencies
State proof project depends on this story.
This story also needs to be done in parallel with updating algokey

Existing integration test that test for proper encoding will fail and will need to be updated to expect msgpack rather than sqlite data file

## Acceptance
Ensure that the updated integration test pass