Methods for getting current URL and host would be useful

A few plugins need the current URL, or the current hostname (Both TOTP and Webauthn needs this).

To avoid code duplication and inconsistencies, core should provide functions to get:

• Current hostname
• Current URL
• Current URL without last part (to build absolute link to other pages)
• Current base URL? (Not easy to find out, but if we want to add url rewrite it will be useful)