Allow for customizable tile cache size

[Swarkin]

As far as I understood the code base, once the LruCache runs out of spaces, older tiles are unloaded from egui? Im not sure what exactly happens after tiles are dropped out of the data structure, are they re-requested entirely or loaded from disk?

https://github.com/podusowski/walkers/blob/839abbf385fd4db6b19a9f69a4492e8e92459b48/walkers/src/http_tiles.rs#L74

It would be beneficial to increase this limit for my use case either way.

[podusowski]

They are downloaded again, unless http cache have them. You can think of this and http cache as a two layered cache. And yes, I know that current http cache kind of sucks. I was thinking that maybe object_store crate gives more control, or at least it reduces amount of code in walkers, but can't really find any spare time to dig into it.