Performance: Use 'If-Modified-Since' header when re-requesting expired tiles

[NathanMOlson]

When a tile expires, it is re-requested. Before this PR, it is always transferred over the network. After this PR, the If-Modified-Since header is set in the re-request, so the server can send a 304 Not Modified response if the tile has not been modified, instead of resending the tile.

This is primarily intended for use with tiles that have a very short expiration time (<10 seconds). This could be useful when the underlying data is changing rapidly.

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Implement for raster tile source.
• Extend to all tile sources (vector, raster-dem).
• Write tests for all new functionality.
• Add an entry to CHANGELOG.md under the ## main section.

[HarelM]

Overall, this is a good idea.
The complexity of data flow and handling of different response types is what you should be looking out for though (for example 404 overzooming, etc...).