feature(mongodb): Added support for MongoDB Go driver v2

[0xshamaiev]

What changed

Adds database/mongodb/v2 for applications using MongoDB Go driver v2,
following the separate versioned-backend pattern used by PGX.
The v2 driver backend is 95% similar to v1, so it's mostly mechanical API changes.

• The existing MongoDB v1 backend, API, and URL schemes are kept in place.
• Added mongodb2:// and mongodb2+srv:// schemes.
• Added the mongodb2 CLI build tag and included it in default builds.
• Document usage and add tests for driver registration, SRV scheme
  handling, shared migration version records, and advisory locking.

Testing

• Lint and all-package compilation passed.
• Core tests and CLI build/startup checks passed.
• MongoDB 5.0–9.0 integration tests passed with race detection.
• Transaction commit and rollback tests passed on MongoDB 5.0.

[0xshamaiev]

@Fontinalis @dhui this should resolve #1368 and doesn't break compatibility for projects that will still use the v1 driver.

[coveralls]

coverage: 55.67%. first build — 0xshamaiev:master into golang-migrate:master

[0xshamaiev]

@Fontinalis , hey, I ran all the checks locally and can confirm they pass. Something is wrong with SQL Server 2025 tests; they must be failing my PR's CI.