Add link to sponsor page

Author: alexrudall

ruby-openai.gemspec

@@ -15,6 +15,7 @@ Gem::Specification.new do |spec|
   spec.metadata["source_code_uri"] = "https://github.com/alexrudall/ruby-openai"
   spec.metadata["changelog_uri"] = "https://github.com/alexrudall/ruby-openai/blob/main/CHANGELOG.md"
   spec.metadata["rubygems_mfa_required"] = "true"
+  spec.metadata["funding_uri"] = "https://github.com/sponsors/alexrudall"
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.