[Constructor] Suggested Fee Dry Run

[patrick-ogrady]

This PR adds support for "dry running" a transaction to get the SuggestedFee without actually signing or broadcasting it. This is useful for amending transactions in UTXO-based blockchains and for ensuring an account has sufficient funds in account-based blockchains.

Changes

• Add dry_run to Broadcast
• Add FindCurrencyAmount to actions (need to parse suggested fees)
• Add Assert to actions where a value can be checked to be positive (otherwise will exit)...ensure we don't create invalid transactions
• implement Assert
• implement FindCurrencyAmount
• implement dry_run
  • existing tests passing

[coveralls]

Pull Request Test Coverage Report for Build 6974

• 85 of 113 (75.22%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 78.123%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
constructor/worker/worker.go	37	41	90.24%
fetcher/construction.go	0	5	0.0%
constructor/coordinator/coordinator.go	32	51	62.75%

Totals
Change from base Build 6902:	0.1%
Covered Lines:	5710
Relevant Lines:	7309

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💛 - Coveralls