Add yaml_decode function to stdlib

[djcode]

River/Grafana Agent has the ability to parse json via the json_decode function.
This PR adds the same functionality for yaml files.

[wildum]

Thanks for adding this, it looks good to me :) Could you just add some tests under vm/vm_stdlb_test.go please?

[wildum]

Note: I tried locally to extract targets from a local yaml file and it worked well:

targets:
  - __address__: "localhost:9001"
  - __address__: "localhost:12345"

local.file "default" {
  filename = "../Downloads/example_yaml_file.yaml"
}

prometheus.scrape "scrape_prom_metrics" {
  targets = yaml_decode(local.file.default.content).targets
  forward_to = [prometheus.remote_write.scrape_prom_metrics.receiver]
}

[djcode]

With the move of River under the Alloy repo, do I need to resubmit this PR? Will this also be moved over and merged?

[wildum]

Sorry I thought this PR was already merged. If you don't mind then yes you can move it to Alloy else I can also do it for you

[mattdurham]

Do you want to move this to the Alloy repo? If not we can do it for you since its a pretty good feature to have in Alloy.

[djcode]

@mattdurham More than happy for you to move it to alloy for me.
I've been inundated with personal distractions recently and haven't had a chance to play with alloy yet.