feat: support write multi fragments or empty fragment in one spark task

[SaintBacchus]

Now FileFragment::create only support create one file fragment and in spark connector will cause these two issues:

1. if the spark task is empty, this api will have exception since there is no data to create the fragment.
2. if the task data stream is very large, it will generate a huge file in lance format. It is not friendly for spark parallism.

So I remove the assigned fragment id and add a new method named FileFragment::create_fragments to generate empty or multi fragments.

[github-actions]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.03937% with 19 lines in your changes missing coverage. Please review.

Project coverage is 78.72%. Comparing base (dc9afbb) to head (0489103).

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/fragment.rs	0.00%	11 Missing ⚠️
rust/lance/src/dataset/fragment/write.rs	94.73%	0 Missing and 6 partials ⚠️
java/core/lance-jni/src/fragment.rs	0.00%	1 Missing ⚠️
java/core/lance-jni/src/utils.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3183      +/-   ##
==========================================
+ Coverage   78.06%   78.72%   +0.66%     
==========================================
  Files         243      243              
  Lines       82661    82793     +132     
  Branches    82661    82793     +132     
==========================================
+ Hits        64529    65180     +651     
+ Misses      14932    14833      -99     
+ Partials     3200     2780     -420     

Flag	Coverage Δ
unittests	78.72% <85.03%> (+0.66%)	⬆️

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