MM-13664 Cache external link metadata when populating post metadata (#10128)

* MM-13664 Added LinkMetadata types

* MM-13664 Use LinkMetadata when populating post metadata

* Fix unused import

* Fix index name on SQLite

* Finish adding unit tests

* Address feedback

* Increase max length of URL column to 2048 characters

store/storetest/mocks/LinkMetadataStore.go

@@ -0,0 +1,46 @@
+// Code generated by mockery v1.0.0. DO NOT EDIT.
+
+// Regenerate this file using `make store-mocks`.
+
+package mocks
+
+import mock "github.com/stretchr/testify/mock"
+import model "github.com/mattermost/mattermost-server/model"
+import store "github.com/mattermost/mattermost-server/store"
+
+// LinkMetadataStore is an autogenerated mock type for the LinkMetadataStore type
+type LinkMetadataStore struct {
+	mock.Mock
+}
+
+// Get provides a mock function with given fields: url, timestamp
+func (_m *LinkMetadataStore) Get(url string, timestamp int64) store.StoreChannel {
+	ret := _m.Called(url, timestamp)
+
+	var r0 store.StoreChannel
+	if rf, ok := ret.Get(0).(func(string, int64) store.StoreChannel); ok {
+		r0 = rf(url, timestamp)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(store.StoreChannel)
+		}
+	}
+
+	return r0
+}
+
+// Save provides a mock function with given fields: linkMetadata
+func (_m *LinkMetadataStore) Save(linkMetadata *model.LinkMetadata) store.StoreChannel {
+	ret := _m.Called(linkMetadata)
+
+	var r0 store.StoreChannel
+	if rf, ok := ret.Get(0).(func(*model.LinkMetadata) store.StoreChannel); ok {
+		r0 = rf(linkMetadata)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(store.StoreChannel)
+		}
+	}
+
+	return r0
+}