stake-pool: Specify the space used for new stake accounts (OS-SSP-SUG-00)

[joncinque]

Description

Ensure rent-exempt calculations are performed on actual data size instead of theoretical data size. E.g.
in AddValidatorToPool, rent exempt calculation on the reserve_pool is performed on the default
account size:

let space = std::mem::size_of::<stake::state::StakeState>();
let stake_minimum_delegation = stake::tools::get_minimum_delegation()?;
let required_lamports = minimum_delegation(stake_minimum_delegation)
.saturating_add(rent.minimum_balance(space));

If it was possible to create a Stake account larger than the default (200 bytes), this computation would
be inaccurate and the stake account may be able to be closed here.
Note: the stake program validates the account size during initialization and during split the remaining
lamports must be either greater than the minimum requirement or zero.

Remediation

Calculate minimum rent based on the actual size of the account to be created.

In all other places, we're using the rent_exempt_reserve reported on the stake account's Meta, so we're safe there.