Auth: Add Sigv4 auth option to datasources (#27552)

* create transport chain

* add frontend

* remove log

* inline field updates

* allow ARN, Credentials + Keys auth in frontend

* configure credentials

* add tests and refactor

* update frontend json field names

* fix tests

* fix comment

* add app config flag

* refactor tests

* add return field for tests

* add flag for UI display

* update comment

* move logic

* fix config

* pass config through props

* update docs

* pr feedback and add docs coverage

* shorten settings filename

* fix imports

* revert docs changes

* remove log line

* wrap up next as round tripper

* only propagate required config

* remove unused import

* remove ARN option and replace with default chain

* make ARN role assume as supplemental

* update docs

* refactor flow

* sign body when necessary

* remove unnecessary wrapper

* remove newline

* Apply suggestions from code review

* PR fixes

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

docs/sources/administration/configuration.md

@@ -649,6 +649,12 @@ Administrators can increase this if they experience OAuth login state mismatch e
 
 Limit of API key seconds to live before expiration. Default is -1 (unlimited).
 
+### sigv4_auth_enabled
+
+> Only available in Grafana 7.3+.
+
+Set to `true` to enable the AWS Signature Version 4 Authentication option for HTTP-based datasources. Default is `false`.
+
 <hr />
 
 ## [auth.anonymous]