Add IMDSv2 support to AwsCandidateHarvester

[dsmeytis]

Hello, AWS recommend to disable IMDSv1 support for EC2 instances: https://aws.amazon.com/blogs/security/get-the-full-benefits-of-imdsv2-and-disable-imdsv1-across-your-aws-infrastructure/
However it breaks AwsCandidateHarvester functionality because REST endpoints it uses to detect EC2 and obtain private/public IPs become unavailable. I would like to propose the fix that will work for both IMDSv1 and IMDSv2:

• doTestEc2() will now try to obtain an API token as EC2_TEST_URL is unavailable without a token.
• added parameters to fetch() to set a method and pass required headers.

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[dsmeytis]

already signed