Add prefix to arrow definitions to avoid conflicts

[sighingnow]

Proposed changes

This pull request addresses two bugs:

• include_directories(BEFORE SYSTEM ${ARROW_INCLUDE_DIR}) will bring arrow_ep/include/... to gar's cmake dependency
• Without a private prefix for arrow definitions, there would be conflicts when the external projects both requires arrow and gar (using add_subdirectory).

Types of changes

What types of changes does your code introduce to GraphAr?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

[github-actions]

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