firebase-ios-sdk

Replace uses of absl::string_view in constructor parameters with std::string (#1676)

* Add a MakeStringView benchmark

Turns out this is worse than just creating a std::string:

BM_MakeString             29 ns         29 ns  189065849
BM_MakeStringView         59 ns         59 ns   93013367

* Convert database_id to take std::string

* Convert DatabaseInfo to take std::string

* Convert User to take std::string

* Convert Token to take std::string

* Convert MakeStringView([NSString stringWithFormat:]) to StringFormat