Raised from #1648

We should extend the spo site set command with a new option called --sharingCapability that is optional, which accepts a valid SharingCapabilities value.

The valid values are as follows.

• Disabled
• ExternalUserSharingOnly
• ExternalUserAndGuestSharing
• ExistingExternalUserSharingOnly

Source: https://docs.microsoft.com/en-us/previous-versions/office/sharepoint-csom/dn174825(v=office.15)