feat: enable multiple sources, render as a mosaic

[hrodmn]

Summary

• We want to be able to support tile requests for datasets that are stored in multiple icechunk stores on distinct spatial grids. We do this type of thing for COGs using titiler.mosaic.MosaicTilerFactory

Testing

• Working locally on a sample of the NAQFC CONUS + AK Icechunk stores:
  • url=s3://naqfc/aqmv7/o3&url=s3://naqfc/aqmv7/o3_ak&variable=ozcon

PR checks

• Standard CI runs automatically on each push.
• To run the CDK synth check, add the run-cdk-checks label to this PR.
• If you push more commits after that run completes, remove and re-add the label to run it again.
• To trigger a dev deployment, add the deploy-dev label. It smoke-tests tiles from the native MUR, virtual MUR, and virtual NLDAS Icechunk stores after deployment.