Replace Get{Signed,Unsigned} with GetInteger<T> and return std::optional
so you can distinguish between the value not being present and it being
explicitly set to the previous fail_value. All existing uses are
replaced by calling value_or(fail_value).
Continuation of #129818
Currently, all request handlers are implemented as free functions in
lldb-dap.cpp. That file has grown to over 5000 lines and is starting to
become hard to maintain. This PR moves the request handlers into their
own class (and file), together with their documentation.
This PR migrates about a third of the request handlers and the rest will
be migrated in subsequent commits. I'm merging this in an incomplete
state because almost any lldb-dap change is going to result in merge
conflicts and migrating request handlers one by one is easier to review.
