[xy] Update the file content API implementation. (mage-ai#5267)

# Description
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Please also include relevant motivation and context.
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Close: mage-ai#4558
Update the file content API implementation to take the whole subpath as
the argument.
When web server proxy automatically decode the url, the file content API
throws error. This PR fixes that.

# How Has This Been Tested?
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- [x] Tested locally with API call
`/api/file_contents/default_repo/data_loaders/file_name.py


# Checklist
- [x] The PR is tagged with proper labels (bug, enhancement, feature,
documentation)
- [x] I have performed a self-review of my own code
- [ ] I have added unit tests that prove my fix is effective or that my
feature works
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation

cc:
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mage_ai/server/server.py

@@ -345,6 +345,7 @@ def make_app(
  ),
  # Download resource
  (r'/api/downloads/(?P<token>[\w/%.-]+)', ApiResourceDownloadHandler),
+ (r'/api/(?P<resource>file_contents)/(?P<pk>.+)', ApiResourceDetailHandler),
  (
  r'/api/(?P<resource>\w+)/(?P<pk>[\w\-\%2f\.]+)'
  r'/(?P<child>\w+)/(?P<child_pk>[\w\-\%2f\.]+)',