GH-48590: [C++] Migrate SFINAE enable_if patterns to C++20 concepts

[abhishek593]

Rationale for this change

We can replace the SFINAE-based enable_if_* patterns with C++20 concepts. This gives us better compiler diagnostics, cleaner code, and faster compile times.

What changes are included in this PR?

Note: I have broken down the migration into several commits so that each commit can be independently reviewed.

• Defined equivalent arrow concepts in type_traits.h that wrap existing is_*_type traits.
• Migrated raw std::enable_if / enable_if_tstd::is_* patterns to use standard and requires.

Are these changes tested?

Yes. No new tests needed since this is a pure refactor; the concept constraints are semantically equivalent to the old enable_if guards.

Are there any user-facing changes?

No functional changes. Template error messages from the compiler will be more readable on type mismatches. The old enable_if_* aliases remain available, and can be cleaned up in a follow-up PR.

• GitHub Issue: [C++] C++20: add/use concepts #48590