MEP for search by primary keys (#23193)

Signed-off-by: Nyah01 <yang.cen@zilliz.com>

docs/design_docs/20230403-search_by_pk.md

+# MEP: Search By Primary Keys
+
+Current state: Under Discussion
+
+ISSUE: [[Feature]: Support to search by primary keys #23184](https://github.com/milvus-io/milvus/issues/23184)
+
+Keywords: Search, ANN
+
+Released: v2.3.0
+
+## Summary
+
+Support to search (ANNS) by the query vectors corresponding to the given primary keys.
+
+## Motivation
+
+For now, Milvus requires passing the query vectors to do anns, we have to fetch the vectors first if they are in the collection, which is complex and slow.
+
+We need a way to do anns directly for the corresponding vectors of given primary keys, which should be more efficient.
+
+## Public Interfaces
+
+Add new field `primary_keys` in `SearchRequest`, all SDKs adds new method:
+```golang
+func SearchByPK(
+    ctx context.Context, 
+    collName string, 
+    partitions []string,
+	expr string, 
+    outputFields []string, 
+    primaryKeys []entity.PrimaryKey, 
+    vectorField string, 
+    metricType entity.MetricType, 
+    topK int, 
+    sp entity.SearchParam, 
+    opts ...SearchQueryOptionFunc,
+) ([]SearchResult, error)
+```
+
+## Design Details
+
+Proxy fetches the vectors by primary keys from QueryNodes first, and then search with these vectors.
+
+For better performance, we will add a new RPC interface for QueryNode:
+```proto
+rpc Fetch(FetchRequest) returns (FetchResponse) {}
+```
+which only supports to fetch by primary keys, this will be more efficient than the present `Query` interface.
+
+
+## Compatibility, Deprecation, and Migration Plan
+
+None
+
+## Test Plan
+
+### Unit Tests
+
+- Test for fetching in segcore
+- Test for fetching in QueryNode
+- Test for searching by primary keys in Proxy
+
+
+### E2E Tests
+|                  Test Cases                  |         Expected Behavior          |
+| :------------------------------------------: | :--------------------------------: |
+|      search by non-existed primary keys      |            report error            |
+|        search by existed primary keys        | return topk results for each query |
+| all present tests cases of search by vectors |              the same              |
+
+## Rejected Alternatives
+
+Implement this without adding the `Fetch` interface, using `Query` to retrieve the vectors and then search by vectors.
+
+## References
+
+None