pkg/metadata: Implement Load interface for files

Store msg+sig in the same file so that it's easier to do atomic read/writes.

Skeleton to start with:

package metadata

import (
        "context"
        "crypto"
        "fmt"
)

// FileSource implements the Loader interface for signed metadata objects that
// are locatable via files.  The following signature schemes are supported:
//
//   - Ed25519 (RFC 8032, §5.1)
//   - RSASSA-PKCS1-V1_5 with SHA256 (RFC 3447, §8.2)
//
// Use NewFileSource() to configure a new file source with FileSourceOptions.
type FileSource struct {
        opts FileSourceOptions
}

// FileSourceOptions are options to configure a file source
type FileSourceOptions struct {
        PublicKey crypto.PublicKey
        FilePath  string
}

// NewFileSource configures a new file source based on the provided options
func NewFileSource(opts FileSourceOptions) FileSource {
        return FileSource{opts: opts}
}

// Load implements the Loader interface
func (s *FileSource) Load(ctx context.Context) ([]byte, []byte, Metadata, error) {
        return nil, nil, Metadata{}, fmt.Errorf("TODO: implement file sources")
}