Redesign codec

Resolves https://github.com/pion/mediadevices/issues/114

* Remove codec registrar
* Completely redesign how codec is being discovered, tuned, and built
* Update examples
* Update unit tests

pkg/codec/opus/params.go

@@ -0,0 +1,30 @@
+package opus
+
+import (
+	"io"
+
+	"github.com/pion/mediadevices/pkg/codec"
+	"github.com/pion/mediadevices/pkg/io/audio"
+	"github.com/pion/mediadevices/pkg/prop"
+	"github.com/pion/webrtc/v2"
+)
+
+// Params stores opus specific encoding parameters.
+type Params struct {
+	codec.BaseParams
+}
+
+// NewParams returns default opus codec specific parameters.
+func NewParams() (Params, error) {
+	return Params{}, nil
+}
+
+// Name represents the codec name
+func (p *Params) Name() string {
+	return webrtc.Opus
+}
+
+// BuildVideoEncoder builds x264 encoder with given params
+func (p *Params) BuildAudioEncoder(r audio.Reader, property prop.Media) (io.ReadCloser, error) {
+	return newEncoder(r, property, *p)
+}