[MLIR] Add Egress dialects maintainers (#151721)
As per https://discourse.llvm.org/t/mlir-project-maintainers/87189, this
PR adds maintainers for the "egress" dialects.
Compared to the original proposal, two changes are included:
* The "mesh" dialect has been renamed to "shard"
(https://discourse.llvm.org/t/mlir-mesh-cleanup-mesh/).
* The "XeVM" dialect has been added
(https://discourse.llvm.org/t/rfc-proposal-for-new-xevm-dialect/).
