Fixed forward declaration of PutUnsafe

[gberninitop]

The PutUnsafe() function can be found via ADL lookup. If ADL lookup rules do not apply, the default implementation of PutUnsafe() is used. The default implementation is a template that should be predeclared in the rapidjson namespace before the first use.

[tencent-adm]

All committers have signed the CLA.

[jeanga]

This PR is required when compiling with latest MSVC (&c++23)