Sendonion: add total amount

[Lagrang3]

Trying to use sendonion in renepay instead of sendpay I noticed that these two rpc's are not compatible
with each other in multi-part payments, ie. if I send a partial payment with sendpay and try to complete
the payment by sending another part with sendonion I get an error because in the internal wallet
sendpay sets the total_amount as specified by the user while sendonion does not making this amount to 0.
Lightningd will only accept further payment parts if the total amounts match. With sendonion there
isn't a field to pass the total amount so we cannot proceed.

With this PR I have added a parameter to sendonion called total_amount_msat in order to make the
sendpay and sendonion interfaces compatible.

[endothermicdev]

The parameter is accepted, but, unless I'm missing something, I think sendonion needs more modification to be used for this application. At least when I tried writing a test to validate this by sending two parts, sendonion comes back with Never attempted payment part 0. I think it would be premature to add the parameter when we can't use or test it yet. Let's wait and get that modification in at the same time. (Or if I'm just messing up the test and it works as is, let me know.)

[Lagrang3]

@endothermicdev, I've added a test.
Thanks for reviewing this.

[Lagrang3]

Rebased and fixed conflicts.

[Lagrang3]

@endothermicdev, help.
I don't know why, but make check-gen-updated is failing.

[endothermicdev]

Ah, just need to run make gen - some of the msggen diff got lost. Nice test.

[rustyrussell]

I suspect this will break pay, which uses sendonion only, and relies on the 0 total_msat?

[endothermicdev]

As this seems to be causing some downstream issues, but nothing is currently depending on it, I'm going to defer to 25.05 at this point.

[Lagrang3]

I suspect this will break pay, which uses sendonion only, and relies on the 0 total_msat?

I will check.

[Lagrang3]

I fixed the backwards compatibility with pay and rebased.

[Lagrang3]

@endothermicdev make gen did not produce any new files for me...