Add changelog URI to gemspec metadata

Added metadata with changelog URI to gemspec

Author: phallstrom

launchdarkly-server-sdk.gemspec

@@ -15,6 +15,10 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/launchdarkly/ruby-server-sdk"
   spec.license       = "Apache-2.0"
 
+  spec.metadata = {
+    "changelog_uri" => "https://github.com/launchdarkly/ruby-server-sdk/blob/main/CHANGELOG.md",
+  }
+
   spec.files         = Dir["lib/**/*.rb", "README.md", "LICENSE.txt"]
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]