thriftbp: Support native slog.LogValuer implementation for errors

The next thrift release, 0.20.0, will add slog.LogValuer for all thrift
generated errors [1] [2]. This prepares us so that WrapBaseplateError
will keep the implementation if it's available to be used with slog.

This also drops support to go 1.20. 1.22 is already at rc2 today so I
think it's time to do that.

[1]: apache/thrift#2884
[2]: https://issues.apache.org/jira/browse/THRIFT-5745

internal/thriftint/baseplate_error.go

@@ -5,6 +5,7 @@ package thriftint
 import (
 	"errors"
 	"fmt"
+	"log/slog"
 	"strings"
 
 	"github.com/apache/thrift/lib/go/thrift"
@@ -36,10 +37,15 @@ var (
 )
 
 type wrappedBaseplateError struct {
-	cause error
-	bpErr baseplateError
+	cause    error
+	bpErr    baseplateError
+	logValue slog.Value
 }
 
+var (
+	_ slog.LogValuer = wrappedBaseplateError{}
+)
+
 func (e wrappedBaseplateError) Error() string {
 	var sb strings.Builder
 	sb.WriteString(fmt.Sprintf("baseplate.Error: %q", e.bpErr.GetMessage()))
@@ -61,6 +67,10 @@ func (e wrappedBaseplateError) Error() string {
 	return sb.String()
 }
 
+func (e wrappedBaseplateError) LogValue() slog.Value {
+	return e.logValue
+}
+
 func (e wrappedBaseplateError) Unwrap() error {
 	return e.cause
 }
@@ -83,11 +93,17 @@ func WrapBaseplateError(e error) error {
 	}
 
 	var bpErr baseplateError
-	if errors.As(e, &bpErr) {
-		return wrappedBaseplateError{
-			cause: e,
-			bpErr: bpErr,
-		}
+	if !errors.As(e, &bpErr) {
+		return e
+	}
+	wrapped := wrappedBaseplateError{
+		cause: e,
+		bpErr: bpErr,
+	}
+	if v, ok := e.(slog.LogValuer); ok {
+		wrapped.logValue = v.LogValue()
+	} else {
+		wrapped.logValue = slog.StringValue(wrapped.Error())
 	}
-	return e
+	return wrapped
 }