Refactor configs to allow unmarshalling instead of accessing them by string paths

[felippeduran]

This PR is separated into two isolated commits:

• The first one refactors configs into nested structs to allow unmarshalling instead of accessing them by string paths
• The second one standardizes constructors to always return pointers instead of values (more aligned with the common use of the new keyword)
  It's recommended to review them independently.

There's an open issue in viper related to the use of env var overrides and unmarshalling, but one of the suggested workarounds is already covered by our code here.

[coveralls]

Pull Request Test Coverage Report for Build 1076509610

• 281 of 387 (72.61%) changed or added relevant lines in 10 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.09%) to 68.245%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
metrics/prometheus_reporter.go	0	2	0.0%
metrics/statsd_reporter.go	1	3	33.33%
config/viper_config.go	38	41	92.68%
worker/worker.go	0	8	0.0%
builder.go	18	28	64.29%
config/config.go	201	282	71.28%

Totals
Change from base Build 1059958609:	1.09%
Covered Lines:	4973
Relevant Lines:	7287

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💛 - Coveralls