Create assembleChildTx to abstract away signing logic in an attempt to better support browser wallets

[matzapata]

Extracted the assembling logic for signingChildTx away into it's separate function to better support browser wallets.

Metamask and other wallets currently don't support signing without sending (issue) and so signChildTx is impossible without users exporting private keys.

This Pr basically renames signChildTx to assembleChildTx removing the final line so the decision of only signing or signing and sending moves back to the developer integrating the SDK. I understand just signing is what makes the most sense but I believe sending doesn't make any harm and allows these extra cases.

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[Flisflas69]

Ok

[Flisflas69]

Ok

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