MangoAccount create from AccountInfo

[bdhobare]

• Moved decoded information from RPC to MangoAccountInfo and created MangoAccount that will hold more methods like account health, liquidation status, etc that can be implemented and tested in subsequent PRs.
• Added two constructors for MangoAccount: using prefetched AccountInfo or by fetching it directly from RPC during construction.
• Tests for these 2 methods

  
  // option 1. Fetch info implicitly from RPC
  const auto& mangoAccount = mango_v3::MangoAccount(pubKey, endpoint);

// option 2 : Use prefetched AccountInfo const auto& mangoAccountInfo = connection.getAccountInfo(pubKey); const auto& mangoAccount = mango_v3::MangoAccount(mangoAccountInfo);

[bdhobare]

@mschneider Simplified MangoAccount to a struct that will have the other methods added. Also replaced the explicit initialization methods with normal constructors. Updated the tests to validate the accountInfo instead of the type.

[mschneider]

included in https://github.com/mschneider/solcpp/pull/27/files