Handle multiple namespaces in mock server

[gupadhyaya]

it is not possible to test different status messages for DALC using our mock server because it does not differentiate namespaces while creating the mock DALC: https://github.com/rollkit/rollkit/blob/main/da/celestia/mock/server.go#L47 This means, all namespace data gets stored into a single key-value store and querying for different namespaces would result in the same data. We need to extend the mock server to create one mock DALC for each namespace.