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

首先定位 issue 中列出的候选枚举的声明和使用位置，包括 ExternalDataSourceType、AuditActionType 和 DatabasePermissionSetValue。在提出范围之前，确定所需的重新设计及其对 SMO 和 SSMS 支持的影响；完成的标准是所选值能够接受服务器返回的新标识符，而无需更新 SMO。