Update login API to v3, use DeriveGeneric, remove aeson-casing dependency

[nvmd]

Hello,

This PR was primarily intended to bring login/logout API (https://spec.matrix.org/v1.17/client-server-api/#post_matrixclientv3login) up-to-date to v3.
As I was going to use GHC generics for FromJson/ToJson instances I’ve also made the relevant fixes in the codebase to keep the usage of it consistent throughout the project.

Full list of changes:

• LoginResponse uses GHC generics for Aeson instances
• Removed aeson-casing: It hasn’t been touched by the author in the last 7 years, several important issues are unresolved, for example Casing edge cases AndrewRademacher/aeson-casing#6 (comment), which is covered by the Aeson’s native camelTo2
• Introduced one single definition of aesonOptions

[TristanCacqueray]

Thanks, nice refactor too!