refactor!: update Kind field to Type in source code (#2312)

Update source code `Kind` to `Type`. It's only changes within our code.
Changes to yaml tag (that will affect users) will be done in later PRs.

This is a breaking change since it updates telemetry's span attribute
from `source_kind` to `source_type`.

Related #817

Future updates will include: 
* Updating a preprocessing function to convert config file from v1 to v2
* Update unmarshal function for ToolsFile to convert config file (test
will fail since the yaml tag is not yet updated).
* Update yaml tag (test will pass).

tests/cloudsql/cloud_sql_create_users_test.go

@@ -35,7 +35,7 @@ import (
 )
 
 var (
-	createUserToolKind = "cloud-sql-create-users"
+	createUserToolType = "cloud-sql-create-users"
 )
 
 type createUsersTransport struct {
@@ -234,7 +234,7 @@ func getCreateUsersToolsConfig() map[string]any {
 		},
 		"tools": map[string]any{
 			"create-user": map[string]any{
-				"kind":   createUserToolKind,
+				"kind":   createUserToolType,
 				"source": "my-cloud-sql-source",
 			},
 		},