Add support for multiple Maven servers

[brunoborges]

Description:
Maven workflows that publish to separate release, snapshot, or private repositories currently have to generate or patch settings.xml themselves. This adds a structured mvn-server-credentials multiline input that generates multiple ordered <server> entries from environment variable names.

The parser rejects malformed entries and duplicate server IDs, while existing single-server inputs and deprecated aliases remain the fallback when the new input is absent. XML escaping, GPG profile behavior, settings overwrite handling, documentation, tests, and checked-in action bundles are updated consistently.

Related issue:
Fixes: #85

Check list:

• Ran npm run check locally (format, lint, build, test) and all checks pass.
• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull request overview

This PR adds first-class support for configuring multiple Maven <server> credential entries in the generated settings.xml, enabling workflows to publish to release/snapshot/private repositories without custom settings.xml patching. It introduces a new multiline input (mvn-server-credentials) that is parsed, validated (malformed entries + duplicate IDs rejected), and used preferentially while preserving the existing single-server inputs (and deprecated aliases) as a fallback.

Changes:

• Add a new mvn-server-credentials multiline input and plumb it through authentication settings generation.
• Implement parsing/validation for multiple server credential entries and update settings.xml generation to emit multiple ordered <server> blocks.
• Update docs, tests, and bundled dist/ artifacts to match the new behavior.

Show a summary per file

File	Description
src/constants.ts	Adds the mvn-server-credentials input constant.
src/auth.ts	Adds parsing + selection logic for multiple servers and updates settings.xml generation to emit multiple <server> entries.
tests/auth.test.ts	Adds coverage for parsing, validation, precedence rules, and multi-server XML output ordering.
action.yml	Documents the new multiline input at the action interface level.
README.md	Adds the new input to the published inputs table.
docs/advanced-usage.md	Adds an “Publishing to multiple Maven servers” section with an example and expected XML output.
dist/setup/index.js	Updates bundled constants export to include the new input.
dist/setup/81.index.js	Updates bundled auth logic for multi-server parsing/generation.
dist/cleanup/index.js	Updates bundled constants to include the new input (unused in cleanup).

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• Files reviewed: 6/9 changed files
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• Review effort level: Lite