Support adding new values to extensible list-type custom tags in SSC issue update during remediation

[jmadhur87]

This PR enhances the SSC issue update command used during remediation to support adding new values to custom tags of type list when the tag is marked as extensible=true using new option --extend.

While verifying for ssc tag update command, found that value updates for non-LIST custom tags (TEXT, DECIMAL, DATE) were incorrectly reported as Updated in fcli, even though SSC made no change.
Added explicit validation and a clear error message so these unsupported value operations now fail fast instead of showing a false success.

Fix: Support adding new values to extensible list-type custom tags in SSC issue update during remediation(#959 )

[rsenden]

Although the class name is somewhat misleading, originally this class only contained helper methods for obtaining the list of custom tags to be assigned/removed to an app version or issue template; it didn't actually do any updates on SSC. The new functionality added in this PR is completely unrelated to the original functionality.

Please refactor:

• Rename the original SSCCustomTagUpdateHelper class (without your changes) to SSCCustomTagAssignmentHelper for better clarity
• Rename the original SSCCustomTagHelper class to SSCCustomTagDefinitionHelper for better clarity
• Move your code for updating custom tag values to SSCCustomTagDefinitionHelper
• Look for opportunities for better code reuse; for example, if it makes sense, utilizing SSCCustomTagDefinitionHelper for creating/updating a custom tag definition, instead of having this implemented directly in the SSCCustomTagCreate/UpdateCommands