DatasourceEditor: Add UI to edit custom HTTP headers (#17846)

* DatasourceEditor: Add UI to edit custom HTTP headers

Support for custom headers was added in #11643 but was missing in the UI.

Fixes #12779

* Review

* Layout updates

Co-authored-by: Dominik Prokop <dominik.prokop@grafana.com>

public/app/plugins/datasource/influxdb/components/__snapshots__/ConfigEditor.test.tsx.snap

@@ -97,7 +97,6 @@ exports[`Render should disable basic auth password input 1`] = `
           labelWidth={10}
           onChange={[Function]}
           onReset={[Function]}
-          placeholder="Password"
           value=""
         />
       </div>
@@ -315,7 +314,6 @@ exports[`Render should hide basic auth fields when switch off 1`] = `
           labelWidth={10}
           onChange={[Function]}
           onReset={[Function]}
-          placeholder="Password"
           value=""
         />
       </div>
@@ -533,7 +531,6 @@ exports[`Render should hide white listed cookies input when browser access chose
           labelWidth={10}
           onChange={[Function]}
           onReset={[Function]}
-          placeholder="Password"
           value=""
         />
       </div>
@@ -751,7 +748,6 @@ exports[`Render should render component 1`] = `
           labelWidth={10}
           onChange={[Function]}
           onReset={[Function]}
-          placeholder="Password"
           value=""
         />
       </div>