Handle API Version with Era switch

[jpraynaud]

Issue

The current API version is handled with a const. We need to be able to make a hot switch when era change (if necessary)

Questions

• Do we need to reboot the HTTP server or a part of it when the era switch requires a change in the routing?
• How to adapt tests with multiple API specs files? (test multiple files without modifying test if not necessary with APISpec)
• How to adapt doc generation with multiple API specs files?
• How to handle API version update on the client (as it does not have access to the Cardano chain)?

Parent issue

#707