GH-51311: [C++][Compute] Add float16 to FloatingPointTypes

[Reranko05]

Rationale

FloatingPointTypes() currently contains only float32() and float64(), so float16() is also omitted from NumericTypes() and PrimitiveTypes().

This was noted as a follow-up during the review of #50513.

Changes

• Add float16() to FloatingPointTypes().
• NumericTypes() and PrimitiveTypes() automatically include float16() through their existing composition from FloatingPointTypes().

Testing

• arrow-type-test passes.

GitHub Issue: #51311

• GitHub Issue: [C++] Make float16 part of FloatingPointTypes() #51311

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[github-actions]

⚠️ GitHub issue #51311 has been automatically assigned in GitHub to PR creator.

[github-actions]

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[Reranko05]

Once #50513 is merged, we can verify that the updated type list works correctly.