Add AWS credential provider support for Bedrock

[jscheid]

What this does

Allows passing Aws::CredentialProvider objects (instance profile, assume role, etc.) instead of static credentials. Provider takes precedence when both are configured. This is a solution to the problem of authenticating with e.g. role-based permissions, as discussed here.

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

Related to #16

[jscheid]

@tpaulshippy here's a PR as discussed, please let me know what you think?

[tpaulshippy]

Looks great! Much nicer than the approach I'm using.