make aws-sdk-rds gem optional

Exit if it's being called for and is not present.

Author: matt-domsch-sp

README.md

@@ -358,6 +358,8 @@ will be fetched as necessary and used for connections until it
 expires.  The value for :host_region will either use the one provided,
 or if not provided, the environment variable AWS_REGION.
 
+You must add the `aws-sdk-rds` gem to your bundle to use this functionality.
+
 | `:use_iam_authentication` | true                                 |
 | ---                       | ---                                  |
 | `:username`               | The database username configured to use IAM Authentication |

lib/mysql2/aws_iam_auth.rb

@@ -1,5 +1,4 @@
 require 'singleton'
-require 'aws-sdk-rds'
 
 module Mysql2
   class AwsTokenAuth
@@ -10,6 +9,13 @@ class AwsTokenAuth
     TOKEN_EXPIRES_IN = (60*14) # 14 minutes
 
     def initialize
+      begin
+        require 'aws-sdk-rds'
+      rescue LoadError
+        puts "gem aws-sdk-rds was not found.  Please add this gem to your bundle to use AWS IAM Authentication."
+        exit
+      end
+
       @mutex = Mutex.new
       # Key identifies a unique set of authentication parameters
       # Value is a Hash

mysql2.gemspec

@@ -24,5 +24,4 @@ Mysql2::GEMSPEC = Gem::Specification.new do |s|
   s.metadata['msys2_mingw_dependencies'] = 'libmariadbclient'
 
   s.add_runtime_dependency 'bigdecimal'
-  s.add_runtime_dependency 'aws-sdk-rds'
 end