Make region field optional in Engine message

[torkins]

The 'region' field in the Engine message construct is meaningless, but in the C# SDK it is required.

Probably the field should be removed from the current service API version entirely if it is meaningless but meanwhile, align the assumptions between client and service by making it optional.

[billscheidel-rai]

Note: This issue has been migrated to https://relationalai.atlassian.net/browse/RAI-7338.

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