feat: Add BrokerClient.close_account

[hiohiohio]

Context:

• close_account was introduced and delete_account was deprecated
  • https://docs.alpaca.markets/reference/post-v1-accounts-account_id-actions-close-1

Changes:

• add BrokerClient.close_account()
• set BrokerClient.delete_account() deprecated. Please use BrokerClient.close_account() instead