feat(ipam): add new mgdb_instance resource

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.98%. Comparing base (5bf53e0) to head (17ac081). Report is 336 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4264 +/- ## ========================================== - Coverage 75.45% 70.98% -4.47% ========================================== Files 202 294 +92 Lines 44323 65970 +21647 ========================================== + Hits 33444 46831 +13387 - Misses 9653 17501 +7848 - Partials 1226 1638 +412 ```

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