Allow explicit Touch ID prompts to be triggered via Go code, which will be used
to guard the (upcoming) credential picker prompt.
To avoid double-prompting users during Touch ID authentication we have to set a
grace period in the underlying LAContext and share it between the functions.
Note that AuthContextGuard (native) uses the LAContext explicitly, whereas
Authenticate (native) uses it through the SecItemCopyMatching query dictionary.
No UX visible changes are made in the PR, despite the fact that prompting occurs
a bit earlier.
#13901
* Allow explicit Touch ID prompts
