Add Bulgaria map to superset-ui-legacy-plugin-chart-country-map

[djok]

This pull request adds Bulgaria map to superset-ui-legacy-plugin-chart-country-map

PR#2@apache-superset/superset-ui-plugins adds geojson

[codecov-io]

Codecov Report

Merging #6938 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6938   +/-   ##
=======================================
  Coverage   63.98%   63.98%           
=======================================
  Files         422      422           
  Lines       20537    20537           
  Branches     2230     2230           
=======================================
  Hits        13141    13141           
  Misses       7264     7264           
  Partials      132      132

Impacted Files	Coverage Δ
superset/assets/src/explore/controls.jsx	42.4% <ø> (ø)	⬆️

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