GH-50457: [C++][Gandiva] Add LN alias for LOG

[lriggs]

Rationale for this change

LN is the standard SQL name for the natural logarithm, while Gandiva exposes it as log. Adding ln as an alias lets SQL engines that emit LN(x) route to the same precompiled log_<type> functions without duplicating any C code. The alias mechanism already exists in Gandiva and is used for pow (alias of power), trunc (alias of truncate), and udfdegrees (alias of degrees).

What changes are included in this PR?

NativeFunction accepts a base_name and a std::vector<std::string> aliases. Each alias gets its own FunctionSignature that points to the same precompiled (pc_name) function — so ln_int32 does not need to exist; the alias "ln" resolves to the same IR symbol log_int32.

The MATH_UNARY_OPS(name, ALIASES) macro in function_registry_math_ops.cc expands to six NativeFunction entries (int32, int64, uint32, uint64, float32, float64), all forwarding to the corresponding log_<type> precompiled symbols.

Are these changes tested?

New unit tests.

Are there any user-facing changes?

Yes, a new Gandiva function.

• GitHub Issue: [C++][Gandiva] Add LN function alias #50457

[github-actions]

⚠️ GitHub issue #50457 has been automatically assigned in GitHub to PR creator.

[dmitry-chirkov-dremio]

Ship it!