[Fix] Workflow scheduling using a String

[antstorm]

Fixes #123

This is a weird issue, but it seems that Object#singleton_class does something to the object and screws up class detection in C extensions (used by Protobufs). Here's an example:

>> a = 'TestWorkflow'
=> "TestWorkflow"
>> a.singleton_class
=> #<Class:#<String:0x000000013320dc10>>
>> Temporal::Api::Common::V1::WorkflowType.new(name: a)
(irb):3:in `initialize': Invalid argument for string field 'name' (given String). (Google::Protobuf::TypeError)
	from (irb):3:in `new'
	from (irb):3:in `<main>'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/lib/bundler/cli/console.rb:19:in `run'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/lib/bundler/cli.rb:504:in `console'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/lib/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/lib/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/lib/bundler/cli.rb:30:in `dispatch'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/lib/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/lib/bundler/cli.rb:24:in `start'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/exe/bundle:49:in `block in <top (required)>'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/lib/bundler/friendly_errors.rb:128:in `with_friendly_errors'
	from /opt/homebrew/Cellar/rbenv/1.1.2/versions/3.0.1/lib/ruby/gems/3.0.0/gems/bundler-2.2.22/exe/bundle:37:in `<top (required)>'
	from /opt/homebrew/opt/rbenv/versions/3.0.1/bin/bundle:23:in `load'
	from /opt/homebrew/opt/rbenv/versions/3.0.1/bin/bundle:23:in `<main>'

Here's the root cause, but I'm not 100% sure of the underlying mechanics — https://twitter.com/ANTStorm/status/1474038270657024001

The fix just calls #singleton_method on a duplicate object avoiding any modifications to the original one.

A new integration test exposing the issue is added.