AVM: Account participation fields

## Problem

Currently, it is not possible in a smart contract to access information about whether an account is online or not and what are its participation key parameters.

## Solution

Add fields about account's participation keys to `acct_params_get`.

## Dependencies

Unknown, but presumably few since majority of account state is already accessible.

## Urgency

Low to medium because not many applications would likely use this. But it would enable applications that would facilitate key delegation, and thus bringing more accounts online, increasing network decentralization, which is a crucial goal.