Chore: Replace new instances of ApiPromises with (re)using interBtcApi instance

[bvotteler]

Resolves #135

Changes made:

• Removed the creation of new instances of ApiPromise when trying to fetch foreign asset data
• Replaced it with using the shared InterBtcApi lib instance instead
• Added tests
• Added a few static structures and helpers to assist testing setups
• Simplified a few other structures to remove unnecessary generic map inversions (that confused Jest)
• Edited test coverage action to store base values when merging to master (enables showing change in coverage percentages)

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@4a8ef15). Click here to learn what that means. The diff coverage is n/a.

@@           Coverage Diff            @@
##             master    #136   +/-   ##
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  Coverage          ?   1.82%           
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  Files             ?      33           
  Lines             ?    2140           
  Branches          ?     484           
========================================
  Hits              ?      39           
  Misses            ?    1973           
  Partials          ?     128           

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