fastbootd: Add getvar max-fetch-size.

Test: run it
Test: see follow up CL on fuzzy_fastboot

Bug: 173654501

Change-Id: I5ed110c5569d83cbe791d04b4abea3a2af2765a9
diff --git a/fastboot/device/commands.cpp b/fastboot/device/commands.cpp
index b2b6a9e..3c56f50 100644
--- a/fastboot/device/commands.cpp
+++ b/fastboot/device/commands.cpp
@@ -136,7 +136,9 @@
             {FB_VAR_DYNAMIC_PARTITION, {GetDynamicPartition, nullptr}},
             {FB_VAR_FIRST_API_LEVEL, {GetFirstApiLevel, nullptr}},
             {FB_VAR_SECURITY_PATCH_LEVEL, {GetSecurityPatchLevel, nullptr}},
-            {FB_VAR_TREBLE_ENABLED, {GetTrebleEnabled, nullptr}}};
+            {FB_VAR_TREBLE_ENABLED, {GetTrebleEnabled, nullptr}},
+            {FB_VAR_MAX_FETCH_SIZE, {GetMaxFetchSize, nullptr}},
+    };
 
     if (args.size() < 2) {
         return device->WriteFail("Missing argument");