enhancement(http provider): allow environment interpolation from http provider config

[sonnens]

Summary

adds a configuration option to the HTTP config provider to interpolate environment variables, similar to config file handling, but defaults to false

Vector configuration

provider:
  type: http
  url: http://whatever
  interpolate_env: true

How did you test this PR?

I have a docker-compose with a configuration service providing configuration to Vector, with remap files pointed to a relative mountpoint ( ie, file: ${REMAPS}/whatever.vrl, docker run -e REMAPS=/path/to/volume )

Change Type

• [ ] Bug fix
• [X] New feature
• [ ] Non-functional (chore, refactoring, docs)
• [ ] Performance

Is this a breaking change?

• [ ] Yes
• [X] No

Does this PR include user facing changes?

• [ ] Yes. Please add a changelog fragment based on our guidelines.
• [X] No. A maintainer will apply the no-changelog label to this PR.

Http config provider is not yet documented, hence no end-user side-effects expected, and the default option is the current behaviour