Support multiple schema of server.json

[pree-dew]

Details mentioned here #488 (comment)

At any point of time, registry should support multiple server.json versions, so that it's not mandatory to update the version, allows flexibility to update when user wants to update.

[tadasant]

The rough idea of what I was thinking here is:

• Publishing process should have an explicit list of supported versions, with logic branches to handle each possibility
• Consumption process should as well

I think we can generally be more strict on publishing than consumption -- requiring an update to push an update feels significantly less heavy-handed than requiring all downstream sub-registry and MCP client consumers to update their implementations.

But maybe there's an argument to keeping them in sync? I think we could build a better experience by decoupling them, but open to other opinions.

[tadasant]

See https://discord.com/channels/1358869848138059966/1369487942862504016/1417915364289020045 for discussion on server.json immutability