Replace some SMO enumerations with string constants

One source of continuous maintenance in SMO is adding new enumeration values as Azure SQL Database and SQL Server evolve over time.
Unless we update the enumerations SMO is unable to generate scripts for objects that use the new values. For the most part, there's very little value in having an enumeration for these identifiers instead of string constants. SMO should be able to store any value returned from the server and use it in script generation without converting it from string to enum value. We can continue to facilitate C# programming by replacing the enumerations with static classes of string constants that have C#-style names for the values SMO knows about.

Here's an incomplete list of candidate enumerations to replace with strings in the next major version bump:

ExternalDataSourceType
ExternalFileFormatType
ExternalStreamingJobStatusType
ExternalTableRejectType
DwTableDistributionType
DwViewDistributionType
ExternalLanguageFilePlatform
AuditDestinationType
OnFailureAction
SensitivityRank
AuditActionType
DatabaseScopedConfigurationOnOff
TemporalHistoryRetentionPeriodUnit
DataRetentionPeriodUnit
CatalogCollationType
AgentSubSystem
JobExecutionStatus

Another enum that needs some re-design to allow SSMS to support new permission values without a SMO update is DatabasePermissionSetValue

まず、issue に記載されている候補列挙型の宣言と使用箇所を確認します。ExternalDataSourceType、AuditActionType、DatabasePermissionSetValue が含まれます。スコープを提案する前に、必要な再設計と SMO および SSMS のサポートへの影響を判断します。選択した値が、SMO の更新なしでサーバーから返される新しい識別子を受け入れられるようになれば完了です。