xds: bootstrap config is not emitted to logs in a human readable way

[easwars]

See the test failure in #7321

Note that the line printing Bootstrap config for creating xds-client is now printing bytes instead of useful human readable config:

grpc-go/internal/xds/bootstrap/bootstrap.go

Line 569 in de51a63

logger.Infof("Bootstrap config for creating xds-client: %+v", config)

This needs fixing.

[easwars]

Fixed by #7347.