Add a function to fetch allowed/available brokers

[prithviraj-gotepatil-algobulls]

• when the user started an RT from pyalogtrading, he gave the name as NUVAMA in the broking details, and the strategy broke.
• Mainly because the user does not know the valid list of names that we have given in AlgoBullsSupportedBrokers
• How do we solve this?
  • One way would be to put the whole list somewhere on the pyalgotrading help site
  • One better way is to give a method like algobulls_connection.get_all_brokers() , this is similar to algobulls_connection.get_all_strategies(), which gives a list of all strategies in the user's account