add support for other GD related servers (including GDPSes)

[yuptune]

this commit adds GDPS_URL to dash-rs, which makes it easier to manage a Pointercrate instance that is used for something other than the main Geometry Dash servers (e.g 1.9 GDPS). If there is anything wrong with this commit, comment on it. Thanks :P

• LG125 & yuptune

[stadust]

I think we can solve this problem using OnceCell:

1. change the const to a static of type OnceCell<&'static str>
2. Have the get_gd_url be a call to OnceCell::get_or_init, with the "or init" part setting it to the boomlings servers
3. use the get_gd_url function in all the submodules of request.

Then if someone wants to use dash-rs with a gdps, they can simply do a OnceCell::set at the beginning of their fn main to update the URL to point to a GDPS.

[yuptune]

Hello, thanks for writing. I have to add other commits to the pull request as well, since there were changes i missed that lg125 put onto the fork. I will do this when I get the changes put on this PR. Thanks ;-)

[yuptune]

Move to the new PR.