Remove optional from FIDO request options (1/2)

This CL makes FIDO request options parameters in the UnmaskDetails and
UnmaskResponseDetails a base::Value::Dict instead of an
std::optional<base::Value::Dict>. This is preferred because
base::Value::Dict's can check if they are empty, and can be cleared, so
it is more readable to use that instead of an optional.

This is CL #1 of 2 in a chain of CLs that are intended to fix the FIDO
enrollment flow. Converting FIDO request options to non-optionals will
make the next CL in the chain much more readable by removing an extra
empty check.

Bug: 40207638
Change-Id: I45117a773493a8b2fd7e16de47c510f3eac5dd8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5542027
Reviewed-by: Olivia Saul <jsaul@google.com>
Commit-Queue: Vinny Persky <vinnypersky@google.com>
Reviewed-by: Siyu An <siyua@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1303462}