New functions: getWithdrawableStake, withdrawSurplusStake and migrateStake #248

/**
 * Gets the amount of withdrawable staked BZZ
 *
 * @param options HTTP request options, such as `headers` or `timeout`
 */
async getWithdrawableStake(options?: BeeRequestOptions): Promise<BZZ>

/**
 * Withdraws ALL surplus staked BZZ to the node wallet.
 *
 * Use the `getWithdrawableStake` method to check how much surplus stake is available.
 *
 * @param options HTTP request options, such as `headers` or `timeout`
 */
async withdrawSurplusStake(options?: BeeRequestOptions): Promise<TransactionId>

/**
 * Withdraws all staked BZZ to the node wallet.
 *
 * **Only available when the staking contract is paused and is in the process of being migrated to a new contract!**
 *
 * @param options HTTP request options, such as `headers` or `timeout`
 */
async migrateStake(options?: BeeRequestOptions): Promise<TransactionId>