Add Bitcoin Gold implementation

[h4x3rotab]

Description

Bitcoin Gold is already added to the trustwallet/assets. This PR adds the implementation including the address codec and transaction signing logic.

Testing instructions

Passes unit tests: ./build/tests/tests tests
Create a new address in walletconsole:

> coin btg
Set active coin to: bitcoingold    Use 'coin' to change.  (name: 'bitcoin gold'  symbol: btg  numericalid: 156)
> addrDefault
Result:  btg1q8hnwn2j2vges5u9h9t456zfd46sjxfmxvk3rvl

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• Prefix PR title with [WIP] if necessary.
• Add tests to cover changes as needed.
• Update documentation as needed.
• If there is a related Issue, mention it in the description (e.g. 'Fixes Adopt Zilliqa to bech32 address format #444 ).

[h4x3rotab]

When signing a BTG transaction, make sure to bitwise or (79 << 8) to the hash_type to enable the proper fork_id.

[codecov]

Codecov Report

Merging #911 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #911   +/-   ##
=======================================
  Coverage   92.51%   92.51%           
=======================================
  Files         418      420    +2     
  Lines       12141    12147    +6     
=======================================
+ Hits        11232    11238    +6     
  Misses        909      909           

Impacted Files	Coverage Δ
src/Bitcoin/Entry.cpp	100.00% <ø> (ø)
src/Bitcoin/Entry.h	100.00% <ø> (ø)
src/BitcoinGold/Address.h	100.00% <100.00%> (ø)
src/BitcoinGold/SegwitAddress.h	100.00% <100.00%> (ø)

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