Add support for extending existing postage stamp capacity

[agazso]

Related and similar to #310

Basically instead of buying a new postage stamp every time the usage threshold is reached with this feature it would be possible to extend the capacity (depth) of the postage stamp using the dilute endpoint.

[agazso]

With dilute you can increase the depth by one and that will double the capacity, but at the same time it will halve the amount. So in order to effectively double the capacity of a postage stamp you need to topup the amount first to double it, and then invoke the dilute to increase the depth (and that will halve the amount).