feat: add Node::[async_]get_direct_child_nodes

[LDeakin]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 8.00000% with 46 lines in your changes missing coverage. Please review.

Project coverage is 81.97%. Comparing base (15358ee) to head (f49338d).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
zarrs/src/node/node_async.rs	0.00%	21 Missing ⚠️
zarrs/src/node/node_sync.rs	18.18%	18 Missing ⚠️
zarrs/src/node.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
+ Coverage   81.72%   81.97%   +0.25%     
==========================================
  Files         166      166              
  Lines       23575    23489      -86     
==========================================
- Hits        19266    19256      -10     
+ Misses       4309     4233      -76     

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