[mlir] Self-nominate for arith dialect maintenance (#157355)

Following https://llvm.org/docs/DeveloperPolicy.html#maintainers, I'd
like to self-nominate for arith dialect maintenance.

As per the policy:
> Maintainers are volunteering to take on the following shared
responsibilities within an area of a project:
> ...

I believe I've been already performing most of the maintenance duties
over the past few years, including direct code contributions, code
reviews, and both starting and participating in relevant RFCs on
discourse. You can look those up with:
* `git log --author=Jakub --oneline --
'mlir/include/mlir/Dialect/Arith*' 'mlir/lib/Dialect/Arith*'`
*
https://github.com/llvm/llvm-project/pulls?q=is%3Apr+label%3Amlir%3Aarith+reviewed-by%3Akuhar
* Some notable RFCs authored:
https://discourse.llvm.org/t/rfc-define-precise-arith-semantics/65507,
https://discourse.llvm.org/t/rfc-poison-semantics-for-mlir/66245,
https://discourse.llvm.org/t/rfc-arith-add-extended-multiplication-ops/66869,
https://discourse.llvm.org/t/rfc-add-integer-add-with-carry-op-to-arith/64573,
https://discourse.llvm.org/t/rfc-arith-should-we-support-scalar-vector-arith-bitcast-s/65427.

In addition to the `core` category maintainers, I can bring additional
perspective as I care both about conversion to llvm (as a user) and to
spirv (as a maintainer).

Author: kuhar

mlir/Maintainers.md

@@ -46,7 +46,7 @@ dialects, build system and language bindings.
 * ‘ptr’ Dialect ([fabianmcg](https://github.com/fabianmcg))
 
 #### Basic Compute Dialects
-* ‘arith’ Dialect (core)
+* ‘arith’ Dialect (core + [kuhar](https://github.com/kuhar))
 * ‘math’ Dialect (core)
 * Rewrite System Dialects (core)
 * Transform Dialect ([martin-luecke](https://github.com/martin-luecke), [ftynse](https://github.com/ftynse), [rolfmorel](https://github.com/rolfmorel))