Add AMBAR music dataset to Lenskit

[sushobhan2024]

issue #1142

• Added amber.py to load data from its path
• Updated the describe.py so imported dataset can be described with the existing describe command

[mdekstrand]

A few things this needs off the bat:

• tests (tagged with @mark.realdata, see the Amazon and Steam load tests for examples). The tests should probably look at it in data/ambar.
• adding to data/convert.py.
• adding to the data conversion CLI test script.

[codecov]

Codecov Report

❌ Patch coverage is 2.43902% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.17%. Comparing base (928f7eb) to head (d9cc10c).

Files with missing lines	Patch %	Lines
src/lenskit/data/sources/ambar.py	0.00%	36 Missing ⚠️
src/lenskit/cli/data/describe.py	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1187      +/-   ##
==========================================
- Coverage   90.37%   90.17%   -0.21%     
==========================================
  Files         261      262       +1     
  Lines       17376    17417      +41     
==========================================
+ Hits        15704    15705       +1     
- Misses       1672     1712      +40     

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