mongodb-flex: add readWriteAnyDatabase role

[PietvanderMeulen]

Description

Adding the readWriteAnyDatabase role for MongDB Flex users.
(It's used by the tf-provider too.)

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see e.g. here)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[rubenhoenle]

CI pipeline is failing because docs aren't up-to-date. Please re-generate the docs by running make generate-docs and commit the result.

[rubenhoenle]

CI pipeline still failing, see comment above.

[rubenhoenle]

@PietvanderMeulen looks good now, again thanks for your contribution!