[crystal][client] support configure method with block

Created by: YusukeHosonuma

Support configure method with block that looks like:

Petstore.configure do |config|
  config.username = "xxx"
  config.password = "xxx"
end

# And likewise...

Petstore::Configuration.new do |config|
  ...
end

config = Petstore::Configuration.new
config.configure do |config|
  ...
end

Note: block_given? is not supported in Crystal currently. https://github.com/crystal-lang/crystal/issues/1013

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