Add extension field to account information

[sb604]

The extension field includes the BIC as a JSON-encoded string. This PR allows to access the extension field via get_information().

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (eb94706) 87.84% compared to head (9e02025) 87.85%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #154   +/-   ##
=======================================
  Coverage   87.84%   87.85%           
=======================================
  Files          26       26           
  Lines        3341     3342    +1     
=======================================
+ Hits         2935     2936    +1     
  Misses        406      406           

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