[Constructor] Inject Public Keys During Transaction Construction

[patrick-ogrady]

Related Issue: coinbase/mesh-cli#112

The v1.4.3 release of the Rosetta Specification allows a Rosetta implementation to request the PublicKeys of any number of AccountIdentifiers for calls to /construction/metadata and /construction/payloads.

This PR adds support for the fetcher to accept PublicKeys in calls to /construction/metadata and /construction/payloads and for the constructor to inject requested public keys automatically.

Changes

• Update fetcher methods for ConstructionPreprocess, ConstructionMetadata, and ConstructionPaylaods
• Inject types.PublicKey during transaction construction in the coordinator
• Add transactional key fetching from key storage

Future PR

• Update rosetta specification to return AccountIdentifier in ConstructionDeriveResponse and SigningPayload (non-breaking by adding field + deprecating)
• Store keys under AccountIdentifier instead of address (there may be multiple keys associated with the same AccountIdentifier.Address)
• Modify broadcast storage to return locked account identifiers instead of addresses

[coveralls]

Pull Request Test Coverage Report for Build 5944

• 13 of 25 (52.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.09%) to 79.402%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
constructor/coordinator/coordinator.go	13	19	68.42%
fetcher/construction.go	0	6	0.0%

Totals
Change from base Build 5889:	-0.09%
Covered Lines:	3454
Relevant Lines:	4350

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💛 - Coveralls