missing method to sign metaTx for commitToBuyerOffer

[levalleux-ludo]

• in packages\core-sdk\src\meta-tx\handler.ts, create the function signMetaTxCommitToBuyerOffer(...) using the model of signMetaTxCommitToOffer(), adapting the parameters according to those required for commitToBuyerOffer (in packages\core-sdk\src\exchanges\handler.ts)
• in packages\core-sdk\src\meta-tx\mixin.ts, create the method signMetaTxCommitToBuyerOffer using the model of signMetaTxCommitToOffer(), calling the function just added at the previous step
• in e2e\tests\meta-tx.test.ts, add some test case for signMetaTxCommitToBuyerOffer (if needed use the buyer initiated test cases from e2e\tests\core-sdk-offers.test.ts as inspiration to create the context required for the signMetaTxCommitToBuyerOffer to work)