feat: per-request OAuth2 auth override

[marinofranz]

Description

This PR implements per-request authentication support, enabling OAuth2 bearer token authentication alongside the existing API key authentication. This is accomplished through a new withAuth() method that allows reusing a single client instance with different credentials per request.

Related Issue

Closes #5

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• ✅ Test update

Changes Made

• Added AuthConfig type supporting both apiKey and oauth2 authentication methods
• Implemented withAuth() method on OpenCloud class for per-request auth
• Made OpenCloudConfig.apiKey optional (was previously required)
• Added authOverride property to HttpClient
• Refactored resource initialization into initializeResources() helper for maintainability
• Updated authentication documentation guide with OAuth2 examples and multi-tenant patterns
• Updated README with quick start examples for both auth methods

Testing

• All existing tests pass (pnpm test)
• Added new tests for new functionality
• Tested manually (describe below)

Code Quality

• My code follows the project's coding style
• I have run pnpm lint and fixed any issues
• I have run pnpm format to format my code
• I have run pnpm typecheck and there are no type errors
• I have reviewed my own code

Documentation

• I have updated the relevant documentation (if applicable)
• I have added/updated JSDoc comments for new/modified code

Additional Context

The proposed API design in #5 was not ideal after researching further and chose the withAuth() pattern over alternative approaches (request-level parameters, middleware, etc.) because it avoids modifying method signatures across all resources

Checklist

• I have read the CONTRIBUTING.md guide
• My PR title follows the Conventional Commits format
• I have rebased my branch on the latest main branch
• I have tested my changes against the Roblox Open Cloud API (if applicable)
• This PR is ready for review

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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