Add Sonatype Central publishing credentials and refactor event handling

[tcheeric]

Summary

Introduce secure credentials for Sonatype Central publishing and perform cleanup on NIP01 event handling APIs. This includes improvements in default sender identity handling and a simplification of event builders.

Related issue: #____

What changed?

• Sonatype Central Credentials: Added configuration for Sonatype Central server credentials using environment variables (CENTRAL_USERNAME and CENTRAL_PASSWORD).
• CI Maven Update: Updated CI configuration to use a custom Maven settings file for consistent builds.
• Default Sender Handling: Updated the NIP01 event builder to synchronize default sender identity when no override is provided.
• API Simplification: Removed sender parameter from event building methods to reduce redundancy and simplify usage.
• Documentation Updates: Adjusted documentation and made necessary changes for these updates.

BREAKING

⚠️ BREAKING:

• sender parameter is removed from several event-building methods.
  • Migration: Remove sender argument from calls to buildTextNote, buildReplaceableEvent, buildEphemeralEvent, and buildAddressableEvent.
• Configuration required for publishing to Sonatype Central (Environment variables must be set).

Review focus

• Confirm secure handling and setup of Sonatype Central credentials.
• Validate simplicity and intuitiveness of the updated event-building API.
• Check for overall consistency in the CI settings updates.

Checklist

• Scope ≤ 300 lines (or split/stack)
• Title is verb + object (e.g., “Refactor auth middleware to async”)
• Description links the issue and answers “why now?”
• BREAKING flagged if needed
• Tests/docs updated (if relevant)