Create Private Network with Specific Keys

## Is your feature request related to a problem? Please describe.

Currently when creating a private network, fresh new accounts with fresh new keys are used.

When using private network for tests, this means that accounts need to be extracted manually first to be used.
While this is feasible, it is not very convenient.

## Describe the solution you'd like.

Allow to specify account address and/or kmd wallet mnemonic and/or account mnemonic in the template file for private network generation.

Ideally the sandbox (https://github.com/algorand/sandbox) should also benefit from such a feature.

## Additional context.

Reach is using a script for that purpose: https://github.com/reach-sh/reach-lang/blob/a3a7c874609b1878a4f5164f462cf32910376ae3/scripts/algorand-devnet/generate_algorand_network.sh

People have been asking about such a feature on Discord.
See:
* https://discord.com/channels/491256308461207573/631209441240416256/808038757035016213
* https://discord.com/channels/491256308461207573/813198189334888469/829363929082888283