Add an alias for `filepath` in Releases and Release Links APIs

What does this MR do and why?

This merge request adds an alias (i.e. direct_asset_path) for filepath field in Releases and Release Links APIs.

filepath is being deprecated and removed in %17.0, but until then, we want to ensure both can be used in the following endpoints:

• Releases API:
  • Create a release
  • Download a release asset
• Release Links API:
  • Create a release link
  • Update a release link

After filepath is removed in #388182 (closed), direct_asset_path should be used solely by customers and users moving forward.

Documentation for the changes introduced here is submitted separately in !108875 (merged).

Resolves #273757 (closed).

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