Integrate with requestSignUp Bridget method in DCAR

[SiAdcock]

The requestSignUp method already exists in Bridget. We should integrate with this when a user requests to sign up to a newsletter. Currently we are making a POST request to the email endpoint.

I expect the corresponding iOS and Android integrations with Bridget are already present. Following this implementation, make sure to test it works in the app on device or in a simulator.

This reimplements the way apps-rendering implemented newsletters sign-up (see #6397)

[shesah]

@shesah to ask Newsletters/Michael C if it's still required