feat: Export GetCodecMap for performance optimization

[asnyatkov]

Exports GetCodecMap to allow building the codec map once and reusing it for multiple lookups.

Before:

for _, pt := range payloadTypes {
    codec, err := sd.GetCodecForPayloadType(pt)  // rebuilds map each time
}

After:

codecMap := sd.GetCodecMap()
for _, pt := range payloadTypes {
    codec := codecMap[pt]
}

Includes benchmark demonstrating the performance improvement.