[PM-17830] Backend changes for admin initiated sponsorships (#5531)

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* Add `Notes` column to `OrganizationSponsorships` table

* Add feature flag to `CreateAdminInitiatedSponsorshipHandler`

* Unit tests for `CreateSponsorshipHandler`

* More tests for `CreateSponsorshipHandler`

* Forgot to add `Notes` column to `OrganizationSponsorships` table in the migration script

* `CreateAdminInitiatedSponsorshipHandler` unit tests

* Fix `CreateSponsorshipCommandTests`

* Encrypt the notes field

* Wrong business logic checking for invalid permissions.

* Wrong business logic checking for invalid permissions.

* Remove design patterns

* duplicate definition in Constants.cs

* Allow rollback

* Fix stored procedures & type

* Fix stored procedures & type

* Properly encapsulating this PR behind its feature flag

* Removed comments

* Updated ValidateSponsorshipCommand to validate admin initiated requirements

---------

Co-authored-by: Conner Turnbull <133619638+cturnbull-bitwarden@users.noreply.github.com>
Co-authored-by: Conner Turnbull <cturnbull@bitwarden.com>

src/Api/Models/Request/Organizations/OrganizationSponsorshipCreateRequestModel.cs

@@ -16,4 +16,14 @@ public class OrganizationSponsorshipCreateRequestModel
 
     [StringLength(256)]
     public string FriendlyName { get; set; }
+
+    /// <summary>
+    /// (optional) The user to target for the sponsorship.
+    /// </summary>
+    /// <remarks>Left empty when creating a sponsorship for the authenticated user.</remarks>
+    public Guid? SponsoringUserId { get; set; }
+
+    [EncryptedString]
+    [EncryptedStringLength(512)]
+    public string Notes { get; set; }
 }