Add prebuild script to create template external_api_keys.dart file

[sneurlax]

Adds a script that creates a template external_api_keys.dart file if it doesn't exist for new contributors that don't have the sidechannel-distributed production API keys and documents its use in README.md

[codecov-commenter]

Codecov Report

Merging #118 (41eacf4) into staging (cb484bf) will decrease coverage by 14.57%.
The diff coverage is n/a.

@@ Coverage Diff @@## staging #118 +/- ##
============================================
- Coverage 39.71% 25.13% -14.58% 
============================================
Files 107 6 -101 Lines 17065 187 -16878 ============================================
- Hits 6777 47 -6730 + Misses 10288 140 -10148 

Impacted Files	Coverage Δ
lib/models/type_adaptors/utxo_model.g.dart	0.00% <0.00%> (-100.00%)	⬇️
lib/models/type_adaptors/lelantus_coin.g.dart	0.00% <0.00%> (-100.00%)	⬇️
...ib/utilities/flutter_secure_storage_interface.dart	30.43% <0.00%> (-69.57%)	⬇️
...verify_recovery_phrase/selected_word_provider.dart
lib/electrumx_rpc/rpc.dart
lib/utilities/text_styles.dart
lib/models/type_adaptors/transactions_model.g.dart
lib/models/paymint/fee_object_model.dart
lib/providers/global/price_provider.dart
lib/providers/ui/home_view_index_provider.dart
... and 94 more

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