added checks for interfaces

Author: asmyasnikov

internal/credentials/access_token.go

@@ -8,6 +8,11 @@ import (
 	"github.com/ydb-platform/ydb-go-sdk/v3/internal/stack"
 )
 
+var (
+	_ Credentials  = (*AccessToken)(nil)
+	_ fmt.Stringer = (*AccessToken)(nil)
+)
+
 // AccessToken implements Credentials interface with static
 // authorization parameters.
 type AccessToken struct {

internal/credentials/anonymous.go

@@ -7,6 +7,11 @@ import (
 	"github.com/ydb-platform/ydb-go-sdk/v3/internal/stack"
 )
 
+var (
+	_ Credentials  = (*Anonymous)(nil)
+	_ fmt.Stringer = (*Anonymous)(nil)
+)
+
 // Anonymous implements Credentials interface with Anonymous access
 type Anonymous struct {
 	sourceInfo string

internal/credentials/static.go

@@ -39,6 +39,11 @@ func NewStaticCredentials(user, password string, config staticCredentialsConfig,
 	}
 }
 
+var (
+	_ Credentials  = (*Static)(nil)
+	_ fmt.Stringer = (*Static)(nil)
+)
+
 // Static implements Credentials interface with static
 // authorization parameters.
 type Static struct {