Returns strings for the default value of a column when a new row is created. The values that could come back:
* `null` when there isn't a default for the column
* a string when there is a default for the column
* a placeholder (currently <TBD>) when the column cannot be updated
* Renaming EditTableMetadata to reflect its SMO source
* Implementation of returning default values
* Unit test for default values
* Reworking column defaults using default constraints
* Adding unit test for new sql script unwrapper
* Disabling flaky test
* Fixing oddities in tests, removing personal tests
* Fixing broken unit test