chore: apply license headers to all the source files

[feng-tao]

Summary of Changes

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Tests

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Documentation

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CheckList

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• PR title addresses the issue accurately and concisely. Example: "Updates the version of Flask to v1.0.2"
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.
• PR includes a summary of changes.
• PR adds unit tests, updates existing unit tests, OR documents why no test additions or modifications are needed.
• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
• PR passes make test

[codecov-commenter]

Codecov Report

Merging #116 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #116   +/-   ##
=======================================
  Coverage   74.92%   74.92%           
=======================================
  Files          18       18           
  Lines         654      654           
  Branches       86       86           
=======================================
  Hits          490      490           
  Misses        135      135           
  Partials       29       29           

Impacted Files	Coverage Δ
search_service/api/base.py	92.30% <ø> (ø)
search_service/api/dashboard.py	100.00% <ø> (ø)
search_service/api/document.py	80.00% <ø> (ø)
search_service/api/table.py	100.00% <ø> (ø)
search_service/api/user.py	40.00% <ø> (ø)
search_service/config.py	100.00% <ø> (ø)
search_service/exception.py	66.66% <ø> (ø)
search_service/models/base.py	72.72% <ø> (ø)
search_service/models/search_result.py	83.33% <ø> (ø)
search_service/models/tag.py	87.50% <ø> (ø)
... and 8 more

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