Change BlutoothManufacturerData.data to base64-encoded tstr

[alexnj]

This PR is changing BluetoothManufacturerData.data from being a bstr and defining it as a base64 encoded text representation as a tstr. Since BiDi is a protocol over JSON, this seems the correct way to represent bytes per RFC 8610.

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[reillyeon]

It seems weird to push how to encode binary data into individual specs. Can we get some guidance from WebDriver BiDi folks on the correct type for this type of argument?

[alexnj]

BiDi spec has one example that encodes binary data, and that seems to refer to forgiving-base64 encode. Perhaps it makes sense for BiDi to (re-)export a definition that allows other specs to link to the correct way to encode binary data.

/cc @OrKoN

[OrKoN]

Yes, using base64 and a text string is currently recommended for binary data if the size is relatively small and can be inline in a protocol message.

[OrKoN]

To clarify, I think referencing the base64 algorithm in the infra spec is sufficient as we do not have a central place to handle binary data in the WebDriver BiDi spec. Also from the client and implementation perspective the format is command/event-specific and we do not expect to change/specify it centrally. If we have a better way in the future, we can re-visit but so far I know know of plans to have a better way to deal with large binary files but I do not think a solution for that would be applicable here since CDP also implements this data as base64-encoded strings.