backend/http: add support for configuration by environment variable

[mattkasa]

Hello!

This PR adds support for configuration of the http backend by environment variables, making it consistent with the majority of the other backends (artifactory, azure, consul, cos, etcdv3, gcs, kubernetes, manta, oss, s3, swift all support environment variables).

This allows for drastically cleaner CI configurations for users who use an http backend for Terraform.

A test for configuration by env has been added, and the docs have been updated to reflect the variables.

Thanks!

[hashicorp-cla]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #25439 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
backend/remote-state/http/backend.go	82.83% <100.00%> (+0.66%)	⬆️
terraform/node_resource_plan.go	91.80% <0.00%> (-1.64%)	⬇️
dag/marshal.go	53.33% <0.00%> (-1.12%)	⬇️

[nicholasklick]

This will fix #22527

[nicholasklick]

@pkolyvas Think this can move forward now?

[mildwonkey]

This is great @mattkasa , thank you!

[ghost]

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