chore: add .mailmap to unify author identities

[alaahong]

What

Adds a repository-root .mailmap that maps a personal address used in some earlier commits onto the corresponding ASF identity:

Zhang Zhe <zhangzhe@apache.org> ian zhang <ian.zhangzhe@gmail.com>

Why

A few commits (for example the RC6 release commit 8f1e1e1) were authored under a personal address, while the release itself is signed and published under the ASF identity zhangzhe@apache.org. .mailmap canonicalizes the author identity for git log / git shortlog / git blame and for the GitHub contributors view, so the public history is attributed consistently.

Scope and safety

• Display-level only: no history rewrite, no force-push. All existing SHAs, PR links and signed tags stay valid.
• Does not touch any already-staged release artifact, the 2.1.0-incubating-rc6 tag, or dist/dev.
• Single added file; no build or runtime impact.

Verification

$ git shortlog -sne HEAD | grep 'Zhang Zhe'
    56  Zhang Zhe <zhangzhe@apache.org>

$ git log --use-mailmap --author='ian.zhangzhe'
(no output - all such commits are folded onto the ASF identity)