Refactoring: create new internal package

[adarsh0728]

• Is the content of metadata.go same in all packages ?

If so, one option to share code is to create a new package internal/metadata/metadata.go (internal/ directory is special in Golang, only this package can import the packages under internal/). The re-export them:

//file: pkg/mapper/metadata.go
import "github.com/numaproj/numaflow-go/internal/metadata"

type SystemMetadata = metadata.SystemMetadata
var NewSystemMetadata = metadata.NewSystemMetadata

Originally posted by @BulkBeing in #208 (comment)

• Also, to not expose internal implementations like shared pkg as public API