Support STAC Alternate Assets in TiTiler

### Description

Some STAC catalogs keep links to cloud-readable assets (S3) in assets > alternate > key > href, rather than the main asset > href - i.e. using the [Alternate Assets STAC extension](https://github.com/stac-extensions/alternate-assets).

We considered adding a mechanism to TiTiler-PgSTAC to use these alternate asset hrefs, but the preferred solution was to implement this at the application level for now.

- https://github.com/stac-utils/titiler-pgstac/discussions/181

## Examples

* https://landsatlook.usgs.gov/stac-server/collections/landsat-c2ard-sr/items/LC09_AK_001011_20240818_20240822_02_SR?.asset=asset-reduced_resolution_browse

```json
"assets": {
"blue": {
"title": "Blue Band (B2)",
"description": "Collection 2 ARD Blue Band (B2) Surface Reflectance",
"type": "image/vnd.stac.geotiff; cloud-optimized=true",
"roles": [
"data"
],
"eo:bands": [
{
"name": "B2",
"common_name": "blue",
"gsd": 30,
"center_wavelength": 0.48
}
],
"href": "https://landsatlook.usgs.gov/tile/collection02/oli-tirs/2024/AK/001/011/LC09_AK_001011_20240818_20240822_02/LC09_AK_001011_20240818_20240822_02_SR_B2.TIF",
"alternate": {
"s3": {
"storage:platform": "AWS",
"storage:requester_pays": true,
"href": "s3://usgs-landsat-ard/collection02/oli-tirs/2024/AK/001/011/LC09_AK_001011_20240818_20240822_02/LC09_AK_001011_20240818_20240822_02_SR_B2.TIF"
}
}
}
}
```
* https://pgstac.demo.cloudferro.com/collections/sentinel-2-l2a/items

```json
"assets": {
"AOT_10m": {
"gsd": 10,
"href": "https://zipper.dataspace.copernicus.eu/odata/v1/Products(fa1f904e-15d2-40da-a4df-8231de3975ea)/Nodes(S2A_MSIL2A_20240328T100631_N0510_R022_T33UVR_20240328T142447.SAFE)/Nodes(GRANULE)/Nodes(L2A_T33UVR_A045779_20240328T100900)/Nodes(IMG_DATA)/Nodes(R10m)/Nodes(T33UVR_20240328T100631_AOT_10m.jp2)/$value",
"type": "image/jp2",
"roles": [
"data",
"sampling:original",
"gsd:10m"
],
"title": "Aerosol optical thickness (AOT) - 10m",
"alternate": {
"s3": {
"href": "s3://eodata/Sentinel-2/MSI/L2A/2024/03/28/S2A_MSIL2A_20240328T100631_N0510_R022_T33UVR_20240328T142447.SAFE/GRANULE/L2A_T33UVR_A045779_20240328T100900/IMG_DATA/R10m/T33UVR_20240328T100631_AOT_10m.jp2",
"storage:tier": "hot",
"storage:region": "waw",
"storage:platform": "CREODIAS",
"storage:requester_pays": false
}
}
}
}
```

## Possible solution

Since we were facing collections like this for VEDA, we implemented a dedicated reader in [veda-backend](https://github.com/NASA-IMPACT/veda-backend/blob/72f446ae95ae67a57dd41dbc5baca59d02a3cfa0/raster_api/runtime/src/alternate_reader.py): `class PgSTACReaderAlt(PgSTACReader)`

VEDA Backend [implemented](https://github.com/NASA-IMPACT/veda-backend/blob/d6aedd75b9751b35c980e85ffc305933e0a87681/raster_api/runtime/src/alternate_reader.py#L10C1-L55C20) a tiler for alternate assets called `s3` this way:

```python
@attr.s
class PgSTACReaderAlt(PgSTACReader):
"""Custom STAC Reader for the alternate asset format used widely by NASA.

Only accept `pystac.Item` as input (while rio_tiler.io.STACReader accepts url or pystac.Item)

"""

def _get_asset_info(self, asset: str) -> AssetInfo:
"""Validate asset names and return asset's url.
Args:
asset (str): STAC asset name.
Returns:
str: STAC asset href.
"""
if asset not in self.assets:
raise InvalidAssetName(f"{asset} is not valid")

asset_info = self.input.assets[asset]
extras = asset_info.extra_fields

if ("alternate" not in extras) or ("s3" not in extras["alternate"]):
raise MissingAssets("No alternate asset found")

info = AssetInfo(url=extras["alternate"]["s3"]["href"], metadata=extras)

info["env"] = {}

if "file:header_size" in asset_info.extra_fields:
h = asset_info.extra_fields["file:header_size"]
info["env"].update({"GDAL_INGESTED_BYTES_AT_OPEN": h})

if requester_pays := extras["alternate"]["s3"].get("storage:requester_pays"):
if requester_pays:
info["env"].update({"AWS_REQUEST_PAYER": "requester"})

if bands := extras.get("raster:bands"):
stats = [
(b["statistics"]["minimum"], b["statistics"]["maximum"])
for b in bands
if {"minimum", "maximum"}.issubset(b.get("statistics", {}))
]
if len(stats) == len(bands):
info["dataset_statistics"] = stats

return info
```

This alternative tiler is then [exposed](https://github.com/NASA-IMPACT/veda-backend/blob/d6aedd75b9751b35c980e85ffc305933e0a87681/raster_api/runtime/src/app.py#L166-L186) at a new endpoint on the same application: `/alt/collections/{collection_id}/items/{item_id}`.

It is up to the user to basically use the right URL pattern for a specific collection.

The new base route `/alt`, as in `/alt/collections`, is non-invasive on the existing routes.

An alternative could be to add a parameter to pass to endpoints like `/collections/{collection_id}/items/{item_id}/tiles/{tileMatrixSetId}/{z}/{x}/{y}.{format}` like `&alternate_assets_key=s3` or so. Or not specify the key but a flag - since `s3` seems to be a commonly used alternate asset key?

## Acceptance criteria

- [ ] The TiTiler tiles endpoints can pick up assets from alternate assets `s3` instead of the main item href, for selected collections

### Labels

- [ ] enhancement
- [ ] discussion
- [ ] help wanted