Handle case where each variable is represented as a separate granule

Right now titiler-cmr can handle the case where granules are defined by a distinct time point and where each granule has the same set of variables. There many datasets in CMR where each granule represents the same timestep but have a different variable. For example, the [Regridded Harmonized World Soil Database v1.2](https://search.earthdata.nasa.gov/search/granules?p=C2216863856-ORNL_CLOUD&pg[0][v]=f&pg[0][gsk]=-start_date&q=netcdf-4&fl=4%2B-%2BGridded%2BModel%2BOutput&tl=1728990006!3!!&zoom=1) dataset has 27 granules that each contain estimates of a different soil property.

To handle this case we could use the `bands_regex` parameter for the `xarray` backend case that would filter the granule results down to one that matches the regex. We would need to change the format of the `mosaic_assets` in this case since the `ZarrReader` can't handle the dictionary of `{band: url}` keys `get_assets` produces when you provide `band_regex`.

I hacked a solution together just to see if it is feasible, here are some tiles from the soil PH layer of that dataset:

```diff
$ git diff titiler/.
diff --git a/titiler/cmr/backend.py b/titiler/cmr/backend.py
index 50307e7..1d2a676 100644
--- a/titiler/cmr/backend.py
+++ b/titiler/cmr/backend.py
@@ -231,12 +231,21 @@ class CMRBackend(BaseBackend):
access=s3_auth_config.access,
bands_regex=bands_regex,
)
-
if not mosaic_assets:
raise NoAssetFoundError(
f"No assets found for tile {tile_z}-{tile_x}-{tile_y}"
)

+ # reformat the mosaic_assets to match expectation for xarray backend
+ # would only want to do this for the backend="xarray" case...
+ if bands_regex:
+ asset = mosaic_assets[0]
+ if len(asset) > 1:
+ raise ValueError("bands_regex returned multiple assets!")
+ url = list(asset["url"].values())[0]
+ provider = asset["provider"]
+ mosaic_assets = [{"url": url, "provider": provider}]
+
def _reader(asset: Asset, x: int, y: int, z: int, **kwargs: Any) -> ImageData:
if (
s3_auth_config.strategy == "environment"
diff --git a/titiler/cmr/factory.py b/titiler/cmr/factory.py
index e10d3af..8e7b73d 100644
--- a/titiler/cmr/factory.py
+++ b/titiler/cmr/factory.py
@@ -121,7 +121,9 @@ def parse_reader_options(

if reader_params.backend == "xarray":
reader = ZarrReader
- read_options = {}
+ read_options = {
+ "bands_regex": rasterio_params.bands_regex,
+ }

options = {
"variable": zarr_params.variable,

```