Update JSON-RPC namespace from 'da' to 'blob'

[randygrok]

Problem

The current implementation uses da as the JSON-RPC namespace, but Celestia Node has deprecated this in favor of blob.

Changes Needed

Client (da/jsonrpc/client.go:263):

"da": &client.DA.Internal,  // Change to "blob"

Server (da/jsonrpc/server.go:106):

srv.rpc.Register("da", apiHandler)  // Change to "blob"

This will change RPC methods from da.GasPrice to blob.GasPrice, etc.

⚠️ Breaking change for anyone using the JSON-RPC API directly.