Adding support for temporary credentials to sigv4 workflows

[sarahzinger]

The primary motivation here is to support Temporary Credentials in Opensearch.

There's some additional work we'll have to do in opensearch before that is fully possible but this is a good step of the way there.

To test locally:

• pull down WIP: add support for temporary credentials for opensearch grafana-aws-sdk-react#66 yarn watch, then yarn link and link in opensearch
• pull down this branch and add replace github.com/grafana/grafana-aws-sdk => /Users/sarahzinger/code/grafana/grafana-aws-sdk to go.mod of opensearch
• (grafana internal only) see provisioning instructions here: https://github.com/grafana/plugin-provisioning/pull/137, take note of comments section with instructions for custom.ini file and .aws/credentials file
• run mage in opensearch
• make run in main grafana
• set up a new datasource as described ^
• save + test is expected to fail because we do not use the same backend flow yet for the health check, so for now ignore
• go to explore and select datasource, metrics should fetch successfully

This really needs tests but I found it difficult to do so. Maybe someone wants to pair with me on it? My attempts lead me to try to a larger refactor project: #103 but it felt like scope creep.

[sarahzinger]

closing as stale, although I think this would work, we can revist if and when we need this. It was mostly a proof of concept.