Feature Request: Public Mode

## Feature Request

#### Is your feature request related to a problem? Please describe.

Running a public CKB node requires several RPC methods to be restricted. This introduces extra work for the operator and extra overhead for a proxy. https://docs.nervos.org/docs/node/run-public-rpc-node

If we want to encourage more community members to run public nodes then we need to make it more accessible. In a perfect world, everyone who needs access would run their own private full node. In realistic observation of our ecosystem and other ecosystems, this does not happen as often as we would like.

#### Describe the solution you'd like

A simple configuration option for public mode that disables RPC methods which should be restricted.

Three configuration states:
- private: All methods enabled for everyone.
- public: All restricted methods are disabled for everyone.
- mixed: Restricted methods are enabled for localhost and disabled for everyone else.

Start with the public RPC node documentation linked in the issue and trace how RPC method restrictions are currently configured. Define the private, public, and mixed behavior, including localhost access in mixed mode. Done means the configuration supports all three states and restricted methods behave accordingly.

Written by the indexing model from the issue text.