cherry-pick: add MongoDB external table scan

[iamlinjunhong]

Add tenant-scoped MongoDB connection DDL and catalog metadata, secure driver-backed scanning, conversion and predicate semantics, downstream aggregation support, and complete ALTER/DROP/account lifecycle handling across CNs. Include one connector E2E CI job, regression coverage, and customer-neutral operational templates.

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #26229

What this PR does / why we need it:

feat: add MongoDB external table scan

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[XuPeng-SH]

Reviewed exact head e71d3de.

No blocking issues found. I audited the 4.2 backport delta against the already-reviewed main implementation, including tenant/catalog generation validation, credential-free plan/RPC serialization, cursor/lease/limiter cleanup, connection retirement races and tombstone pruning, operator Reset/Free/remote round-trip, DDL/account/upgrade lifecycle, conservative predicate pushdown with MO residuals, and BSON conversion/nullability boundaries.

Local verification passed: exact-head make build; full mongodb and mongoscan tests (also under -race); Mongo-focused planner, compile, frontend, CN, bootstrap, parser and config tests; and the backported fill/timewin/aggexec package tests. The PR's MongoDB and general CI checks are green at this head.