Feat/add elastic search api key authentication

[guipal]

Add apiKey authentication method to elasticSearch target

Problem

Currently, the only available method to authenticate elasticSearch target is by using username/password in a Basic Http Auth Method.
This is a limitation and implies using login credentials from a user in the integration

Proposed Implementation

Adding the possibility to configure ApiKey authentication so the integration can be achieved by using an user personal apiKey instead of actual user credentials.

[fjogeleit]

thanks for your contribution.

[coveralls]

Pull Request Test Coverage Report for Build 7167601966

• 10 of 11 (90.91%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 91.999%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/target/elasticsearch/elasticsearch.go	2	3	66.67%

Files with Coverage Reduction	New Missed Lines	%
pkg/target/elasticsearch/elasticsearch.go	1	91.11%

Totals
Change from base Build 7158161642:	0.02%
Covered Lines:	4542
Relevant Lines:	4937

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💛 - Coveralls