Add handler for local output mode

[stephen-soltesz]

This change adds a new handler to the passive mode ETL worker to support v2 datatypes including during local development.

This change adds a new resource, /v2/worker, that is unconditionally available when the worker is started. The /v2/worker handler accepts a filename= parameter (like the existing /worker handler used by the legacy pipeline). However, the /v2/worker only supports v2 datatypes.

With this handler, a developer can run the worker in "local" output mode to test behavior of a new parser without depending on BigQuery or other GCP services (other than the source archive in GCS). This handler also allows adhoc tests in GKE.

Instructions for use are now included in the README.md

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 6470

• 29 of 37 (78.38%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 63.333%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/etl_worker/etl_worker.go	29	37	78.38%
<!--	Total:	29	37	78.38%	-->

Totals
Change from base Build 6456:	0.6%
Covered Lines:	3572
Relevant Lines:	5640

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💛 - Coveralls