Register TogetherAI provider and add chat capabilities

[yangtheman]

What this does

It adds support for Together AI as provider. Together hosts many open source models with various capabilities, but this PR focuses on support only chat capabilities for now. Once this PR gets approved and merged, I will create additional PRs to include additional capabilities.

Re: VCR. I want to run and create VCR cassettes for TogetherAI, but when I try to run it, it looks like I need to run models rake task with real TogetherAI API key. When I do that and run the vcr rake task with bundle exec rake 'vcr:record[togetherai]', it will create cassettes TogetherAI, but those tasks for other providers fail since those were removed whenmodels rake task was run and I don't have API keys for them. Could someone please advise what is the correct way of running vcr rake task for new provider like TogetherAI?

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

Related issues

Relatd to #511