Depend on Tahoe-Capabilities for Tahoe-LAFS capability interactions

[exarkun]

This begins to replace the use of str to represent Tahoe-LAFS data capabilities with use of the tahoe-capabilities library. Specifically, it changes the signatures:

• _zkapauthorizer.tahoe.download - now takes cap as a ReadCapability instead of `str
• _zkapauthorizer.tahoe.ITahoeClient.download - similar. Also, this method no longer takes the extra child_path argument. Instead, there is a new module-scope function download_child which has this functionality.
• _zkapauthorizer.tahoe.Tahoe.download - similar.
• _zkapauthorizer.tahoe.MemoryGrid.download - similar.
• _zkapauthorizer.tahoe._MemoryTahoe.download - similar.
• Consumers of these APIs are updated to deal with the consequences, sometimes with their own public-interface consequences.

[codecov[bot]]

Codecov Report

Merging #419 (b5c2995) into main (8c1185f) will decrease coverage by 0.11%. The diff coverage is 93.70%.

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
- Coverage   96.27%   96.16%   -0.12%     
==========================================
  Files          63       63              
  Lines        7526     7557      +31     
  Branches     1012     1013       +1     
==========================================
+ Hits         7246     7267      +21     
- Misses        208      218      +10     
  Partials       72       72              

Impacted Files	Coverage Δ
src/_zkapauthorizer/tahoe.py	91.36% <82.05%> (-2.00%)	:arrow_down:
src/_zkapauthorizer/tests/test_tahoe.py	98.68% <94.44%> (-0.85%)	:arrow_down:
src/_zkapauthorizer/_plugin.py	96.73% <100.00%> (ø)
src/_zkapauthorizer/recover.py	97.69% <100.00%> (+0.03%)	:arrow_up:
src/_zkapauthorizer/replicate.py	95.61% <100.00%> (-0.02%)	:arrow_down:
src/_zkapauthorizer/resource.py	99.26% <100.00%> (+<0.01%)	:arrow_up:
src/_zkapauthorizer/tests/test_client_resource.py	98.02% <100.00%> (-0.21%)	:arrow_down:
src/_zkapauthorizer/tests/test_plugin.py	99.70% <100.00%> (+<0.01%)	:arrow_up:
src/_zkapauthorizer/tests/test_recover.py	99.10% <100.00%> (+<0.01%)	:arrow_up:
src/_zkapauthorizer/tests/test_replicate.py	93.92% <100.00%> (+0.01%)	:arrow_up:
... and 6 more

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