feat(convars): add GET_CONVAR_FLOAT

[AvarianKnight]

Currently, there's no way to get a convar as a float

This leads to some really hacking solutions (like here)

[blattersturm]

I'm slightly confused as to why we apparently have two sets of convar getter implementations that only differ in slight ways. Marking this one for manual work.

[AvarianKnight]

Doesn't look like client convars was originally a thing (https://github.com/citizenfx/fivem/commit/af718ea4f67f9d7853ca8920a94d5bd2d259619e#diff-41ce82e7751a7ca63df92aaa62f2a4bd92f9b513ef487fb3b36837f6f187974b and https://forum.cfx.re/t/get-convar-and-get-convar-int-client-side/164859)