return user's own username link handle in /whoami response

service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java

@@ -252,6 +252,7 @@ public class AccountController {
         auth.getAccount().getNumber(),
         auth.getAccount().getPhoneNumberIdentifier(),
         auth.getAccount().getUsernameHash().filter(h -> h.length > 0).orElse(null),
+        auth.getAccount().getUsernameLinkHandle(),
         auth.getAccount().isStorageSupported());
   }
 

service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountControllerV2.java

@@ -150,6 +150,7 @@ public class AccountControllerV2 {
           updatedAccount.getNumber(),
           updatedAccount.getPhoneNumberIdentifier(),
           updatedAccount.getUsernameHash().orElse(null),
+          updatedAccount.getUsernameLinkHandle(),
           updatedAccount.isStorageSupported());
     } catch (MismatchedDevicesException e) {
       throw new WebApplicationException(Response.status(409)
@@ -208,6 +209,7 @@ public class AccountControllerV2 {
           updatedAccount.getNumber(),
           updatedAccount.getPhoneNumberIdentifier(),
           updatedAccount.getUsernameHash().orElse(null),
+          updatedAccount.getUsernameLinkHandle(),
           updatedAccount.isStorageSupported());
     } catch (MismatchedDevicesException e) {
       throw new WebApplicationException(Response.status(409)

service/src/main/java/org/whispersystems/textsecuregcm/controllers/RegistrationController.java

@@ -170,6 +170,7 @@ public class RegistrationController {
         account.getNumber(),
         account.getPhoneNumberIdentifier(),
         account.getUsernameHash().orElse(null),
+        account.getUsernameLinkHandle(),
         existingAccount.map(Account::isStorageSupported).orElse(false));
   }
 

service/src/main/java/org/whispersystems/textsecuregcm/entities/AccountIdentityResponse.java

@@ -27,6 +27,9 @@ public record AccountIdentityResponse(
     @JsonDeserialize(using = ByteArrayBase64UrlAdapter.Deserializing.class)
     @Nullable byte[] usernameHash,
 
+    @Schema(description="this account's username link handle, if set")
+    @Nullable UUID usernameLinkHandle,
+
     @Schema(description="whether any of this account's devices support storage")
     boolean storageCapable) {
 }