Add yesod test getCookies utility

[josephsumabat]

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddocks for new, public APIs

Recently was trying to write a test which required that I clear cookies and set them again later on in the test.

cookies <- YT.getCookies
YT.clearCookies

<do something with cookies after>....

Unfortunately getRequestCookies only allows you to access the cookies in a RequestBuilder site context since the type of request is RequestBuilder site () -> YesodExample site () thus something like

cookies <- request $ do
  getRequestCookies

unfortunately does not typecheck and you can only access the cookies from within the request builder. This PR adds the utility funciton getCookies which simply enables the first use case allowing cookies to be accessed outside of a request context without resorting to using an IORef

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[parsonsmatt]

lgtm! do you mind updating the cabal file to make an easy releate?

Thanks!