fix: Accept config to be through file or env vars

[carlosvdr]

fixes #337

[carlosvdr]

So I didnt added it, as in the validation it should fail so didnt saw a reason to add it.

But maybe also add to first validate the env vars, and if they are not all okay to error out a file name is alaso required?

[coveralls]

Pull Request Test Coverage Report for Build 11240998283

Details

• 17 of 30 (56.67%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 70.515%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
config/src/config.rs	17	18	94.44%
service/src/service.rs	0	5	0.0%
tap-agent/src/config.rs	0	7	0.0%

Files with Coverage Reduction	New Missed Lines	%
tap-agent/src/config.rs	1	4.67%

Totals
Change from base Build 11236095653:	-0.06%
Covered Lines:	4556
Relevant Lines:	6461

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💛 - Coveralls

[gusinacio]

LGTM