Add rawBytes to BarcodeValue

[the-mars-rover]

@bharat-biradar Hi there :)

In short, this PR adds rawBytes to the BarcodeValue class.

I'll provide some background to motivate why this would be a valuable addition. I had a requirement when scanning barcodes not encoded using UTF8. The only way I code extract valuable information from these barcodes was through accessing the raw bytes of the barcodes. Hence, I raised an issue and opened a PR on the firebase_ml_vision plugin, please see the links below for context:

• [firebase_ml_vision] Add rawBytes to the Barcode class firebase/flutterfire#1816
• feat(firebase_ml_vision): Add rawBytes to the Barcode class (#1816) firebase/flutterfire#2368

Aside from myself, there are quite a few others who also wanted to see this feature go in (judging by the activity on the Issue and the PR). However, since the firebase_ml_vision was discontinued, the PR was closed and I was directed to use this plugin instead. Consequently I'd like to propose adding this feature :-)

I have tested the change on iOS and Android devices and both are working. Please let me know if there is anything I am missing or you would like me to add :-)

Also, since this is quite a small change I have not raised an issue for it, but will be happy to do so on request. Thanks in advance for your help.

[bharat-biradar]

@marcus-bornman Thank you for the contribution and sorry for the delay.

[the-mars-rover]

@bharat-biradar thank you 🙏 very much appreciated.