feat: allow specific OWNERS reviewers to be excluded from auto-review-request while still counting toward lgtm #1218

Feature Request

(Not a bug — current behavior is working as designed, this is a request for a
new opt-out capability.)

Problem

Every entry in an OWNERS file's reviewers: list is treated identically by
OwnersFileHandler:

• Auto-requested as a GitHub PR reviewer via create_review_request() on every
  PR whose changed files match that OWNERS file's scope
  (assign_reviewers() in owners_files_handler.py)
• Eligible to have their /lgtm (or native GitHub "Approve" review) count
  toward minimum-lgtm for that same scope (_check_if_pr_approved() in
  pull_request_handler.py)

Both behaviors are driven by the same aggregated list
(all_pull_request_reviewers / root_reviewers), so there is no way to keep
one without the other.

For maintainers listed as reviewers in a repo-root OWNERS file, this means
being auto-added as a requested reviewer on every PR in the repo, with no
way to opt out without also losing the ability to /lgtm PRs repo-wide.

Request

Add a way, configurable per reviewer name (not a blanket per-file toggle),
to exclude a specific person from auto-review-request while keeping their
/lgtm eligible to count toward minimum-lgtm for that OWNERS file's scope.

Desired behavior for an opted-out name at the root OWNERS level:

• ✅ Can /lgtm (or GitHub "Approve") any PR in the repo, and it counts toward
  minimum-lgtm, same as today
• ❌ Not included in create_review_request() when only root scope applies
• ✅ (if possible) Still auto-assigned as a requested reviewer when a
  more specific, smaller-scope OWNERS file (e.g. a subdirectory) lists
  the same name without the opt-out flag — i.e. the opt-out should only
  suppress auto-assignment at the level(s) where it's actually set, not
  globally across every OWNERS file that happens to include that name.

Proposed syntax (name-specific, not file-wide)

# root OWNERS
approvers:
  - alice
reviewers:
  - alice
  - bob
  - name: carol
    auto-assign: false   # carol: lgtm counts, but not auto-requested at this scope

# subdir/OWNERS (no opt-out here — carol still gets auto-requested for this scope)
reviewers:
  - carol

Alternative, simpler-to-implement syntax if per-entry objects are undesirable:

reviewers:
  - alice
  - bob
no-auto-assign-reviewers:   # subset of `reviewers`, name-specific opt-out
  - carol

Either way, carol remains in all_pull_request_reviewers (lgtm-counting) for
this file's scope, but is excluded from the create_review_request() call in
assign_reviewers().