Supporting content negotiation via `Accept`

**Describe the bug** As asked in https://github.com/maplibre/maplibre-style-spec/pull/1251#discussion_r2289493068 it is currently not possible to support MLT in tile servers currently. This is because the tile server does not know if MVT or MLT should be returned. This is a simple fix, just including the `Accept` header in the request for tiles. It is a bit more complicated, because the cache/offline mode is not currently MVT/MLT aware and likely should, but that is a different issue. **To Reproduce** Steps to reproduce the behavior: 1. No `Accept` header 1. Don't know if MLT or MVT should be served 1. Tileserver needs to assume MVT for backwards compatability **Expected behavior** `Accept` header