Support setting custom HTTP headers before Map initialization

Nobody has claimed this yet.

We can set custom HTTP headers via setCustomHeaders(...), but we can only do that inside the onMapCreated callback (after obtaining the MapboxMap instance).
However, this timing is too late: the initial network requests (style / glyph / sprite / tile) are already fired before setCustomHeaders done, so those first requests never include our custom headers.

We’d like to define custom headers before any Maps API object is constructed — similar to how we set the access token with MapboxOptions.setAccessToken(...).

Rationale

Some servers require custom auth headers for all map traffic

Enterprise / B2B applications often need to identify the app / org via headers

Prevents the “first request goes out without headers” problem

Thanks!

Start by tracing setCustomHeaders(...) from the onMapCreated callback and compare it with the earlier MapboxOptions.setAccessToken(...) configuration path. Identify where the Maps API object is constructed and determine how headers must be configured before the initial style, glyph, sprite, and tile requests. Done means custom headers are available on those first requests.

Written by the indexing model from the issue text.