[#310] *: Implement string converters for enumerations

Implement `String` / `FromString` method pair in all levels of enum
definitions. From now `String()` returns canonical protojson-compatible
values.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

v2/acl/string.go

@@ -0,0 +1,110 @@
+package acl
+
+import (
+	acl "github.com/nspcc-dev/neofs-api-go/v2/acl/grpc"
+)
+
+// String returns string representation of Action.
+func (x Action) String() string {
+	return ActionToGRPCField(x).String()
+}
+
+// FromString parses Action from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (x *Action) FromString(s string) bool {
+	var g acl.Action
+
+	ok := g.FromString(s)
+
+	if ok {
+		*x = ActionFromGRPCField(g)
+	}
+
+	return ok
+}
+
+// String returns string representation of Role.
+func (x Role) String() string {
+	return RoleToGRPCField(x).String()
+}
+
+// FromString parses Role from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (x *Role) FromString(s string) bool {
+	var g acl.Role
+
+	ok := g.FromString(s)
+
+	if ok {
+		*x = RoleFromGRPCField(g)
+	}
+
+	return ok
+}
+
+// String returns string representation of Operation.
+func (x Operation) String() string {
+	return OperationToGRPCField(x).String()
+}
+
+// FromString parses Operation from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (x *Operation) FromString(s string) bool {
+	var g acl.Operation
+
+	ok := g.FromString(s)
+
+	if ok {
+		*x = OperationFromGRPCField(g)
+	}
+
+	return ok
+}
+
+// String returns string representation of MatchType.
+func (x MatchType) String() string {
+	return MatchTypeToGRPCField(x).String()
+}
+
+// FromString parses MatchType from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (x *MatchType) FromString(s string) bool {
+	var g acl.MatchType
+
+	ok := g.FromString(s)
+
+	if ok {
+		*x = MatchTypeFromGRPCField(g)
+	}
+
+	return ok
+}
+
+// String returns string representation of HeaderType.
+func (x HeaderType) String() string {
+	return HeaderTypeToGRPCField(x).String()
+}
+
+// FromString parses HeaderType from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (x *HeaderType) FromString(s string) bool {
+	var g acl.HeaderType
+
+	ok := g.FromString(s)
+
+	if ok {
+		*x = HeaderTypeFromGRPCField(g)
+	}
+
+	return ok
+}