[feature]: add a default value for timeout_seconds in SendPaymentV2 routerrpc.SendPaymentRequest

[ZZiigguurraatt]

I propose that in the SendPaymentV2 RPC call's routerrpc.SendPaymentRequest that we change the timeout_seconds to have a default value of 60 seconds. Right now, there is no default, so the RPC call results in an error if that field is not populated. Is there a technical reason that timeout_seconds has no default value?

Also, the API docs say timeout_seconds is

An upper limit on the amount of time we should spend when
attempting to fulfill the payment. This is expressed in seconds.
If we cannot make a successful payment within this time
frame, an error will be returned. This field must be non-zero.

However, is this really an "upper limit" if a node along the route stalls for a while during the last payment attempt? Seems like it is more like the time in which the last HTCL will be initiated?

[kelvinator07]

@ZZiigguurraatt @ziggie1984 I'd like to work on this issue.