[skip ci] add enhancement issue template (#3585)

Signed-off-by: Nyudong.cai <yudong.cai@zilliz.com>

.github/ISSUE_TEMPLATE/enhancement.md

+---
+name: "Enhancement request"
+about: Suggest an enhancement idea for Milvus
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Please state your issue using the following template and, most importantly, in English.**
+
+**What would you like to be added ?**
+
+**Why is this needed ?**
+