Refine handling of special indexing status or config states

[lightwalker-eth]

A number of our APIs should be "gated" by config checks and indexing status checks.

For example: some APIs require certain plugins to be activated. If they are not activated, we should return a HTTP 501 error.

Some APIs require indexing status to be following or completed. If the latest indexing status does not meet this then they should return a HTTP 503 error.

See related comments here:

1. https://github.com/namehash/ensnode/pull/1338/files#r2638148049
2. https://github.com/namehash/ensnode/pull/1338/files#r2638156592