feat: choose http method for datasource test

[gjed]

What this PR does / why we need it:

Added option in the datasource config editor for choosing http method to test the datasource with

Which issue(s) this PR fixes:

Issue #279

Fixes #279

Special notes for your reviewer: Hi, just wanted to say that this is my first experience in participating to the Grafana ecosystem. I would really love to contribute and this seems to me a good place to start. I don't really have much experience in js/ts but I'm eager to learn. I'll appreciate any feedback. Thanks,

gjed

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