adding a namespace-like support

[Xuanwo]

adding a namespace-like configuration may make operations on a multi-tenant deployment easier:

1. cache keys can be isolated in a different namespaces
2. we can collect some info or restrict usage at the namespace level, like qps, memory limit, rate limit, etc.

Originally posted by @flaneur2020 in https://github.com/datafuselabs/databend/issues/6786#issuecomment-1193869119