DAC: Introduce experimental (but binding) `V0` API

Part of #5437 - Versioning DAC API.

We want to prefix all the existing endpoints with experimental, but binding v0 API version first. This version will be used by the 1M/tps demo. It also includes the legacy endpoints specific to the Legacy mode. This is important as it decouples the work on the Demo.

As such the first experimental release of the DAC API will look like the following:

• POST v0/store_preimage (Legacy mode only)
• GET v0/verify_signature (Legacy mode only)
• PUT v0/preimage (Coordinator and Legacy mode)
• GET v0/preimage (Coordinator, Committee_member, Observer, Legacy mode)
• PUT v0/dac_member_signature (Coordinator and Legacy mode)
• GET v0/certificates (Coordinator and Legacy mode)
• GET v0/missing_page (Observer and Legacy mode)
• GET v0/monitor/certificate (Coordinator and Legacy mode)
• GET v0/monitor/root_hashes (Coordinator and Legacy mode)