[ ClarkQAQ/nostr ] refactor: extract HttpRequestBaseURL to utils

- Move getBaseURL logic from Relay and BlossomServer to nostr.HttpRequestBaseURL
- Remove redundant ServiceURL field and getBaseURL methods
https://github.com/ClarkQAQ/nostr/commit/d0df613721d8fd5f81b1e2e4af4338b8cbd2c183