Expose `minimumValidityBlocks` in chainstate endpoint from Postage Stamp contract

#### 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

