Types: Use bitcoin secret key wherever possible

[TheCharlatan]

This simplifies the typing and allows us to use proper types instead of raw 32 byte arrays for the Bitcoin secret keys.

[codecov-commenter]

Codecov Report

Merging #600 (c3fd91a) into main (9fabb42) will increase coverage by 0.0%.
The diff coverage is 0.0%.

@@          Coverage Diff          @@
##            main    #600   +/-   ##
=====================================
  Coverage    9.3%    9.3%           
=====================================
  Files         32      32           
  Lines      10621   10616    -5     
=====================================
  Hits         987     987           
+ Misses      9634    9629    -5     

Impacted Files	Coverage Δ
src/cli/command.rs	0.0% <0.0%> (ø)
src/databased/runtime.rs	49.2% <0.0%> (+0.3%)	⬆️
src/rpc/request.rs	2.3% <0.0%> (ø)
src/syncerd/bitcoin_syncer.rs	0.0% <0.0%> (ø)
src/syncerd/types.rs	25.0% <0.0%> (ø)
src/walletd/runtime.rs	0.0% <0.0%> (ø)

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