MINOR: log: explicit typecasting for logformat nodes

MINOR: log: explicit typecasting for logformat nodes

Add the ability to manually specify desired output type after a custom
field name for logformat nodes. Forcing the type can be useful to ensure
value is stored with the proper type representation. (i.e.: forcing
numerical to string to work around the limited resolution of JS number
types)

By default, type is set to SMP_T_SAME, which means the original type will
be preserved.

Currently supported types are: bool, str, sint