Add `MultiRasterSource.from_stac()` constructor

[AdeelH]

Overview

This PR adds a .from_stac() constructor to MultiRasterSource, as an alternative to XarraySource.from_stac(). This constructor creates a RasterioSource for each asset and puts all raster sources together into a MultiRasterSource.

Checklist

• [x] Added unit tests, if applicable
• [x] Updated documentation, if applicable
• [x] Added needs-backport label if the change should be back-ported to the previous release
• [x] PR has a name that won't get you publicly shamed for vagueness

Notes

Testing Instructions

See new unit tests.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.10%. Comparing base (4a613e8) to head (f75fe95).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2156 +/- ## ========================================== + Coverage 90.08% 90.10% +0.01% ========================================== Files 197 197 Lines 9855 9874 +19 ========================================== + Hits 8878 8897 +19 Misses 977 977 ```

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