Expose `minimumValidityBlocks` in chainstate endpoint from Postage Stamp contract

[Cafe137]

Summary

Extend /chainstate endpoint response with a number for minimumValidityBlocks

Motivation

Clients would have all required values for stamp calculations from one endpoint, no need to read contract

Implementation

c.getProperty(ctx, "minimumValidityBlocks", &minimumValidityBlocks)

Drawbacks