chore(input): merge `get_input` and `set_input` into `Elgg_Http_Request`

Fixes #7091 

TODO:
 - [x] Incorporate `Elgg\Context` service from #7101 
 - [x] Incorporate `Elgg_PluginHooksService`
 - [x] Ditch all the duplicated Symfony logic and inject their request class instead.
 - [ ] Implement incomplete tests
 - [ ] Include tests for the global functions that wrap this service
 - [x] Switch to namespaces (#7104)