Feature: Sum Function

[noahmercado]

Reference: #17239

@alisdair
Continuation of closed PR -> #24356

This feature adds an additional collection function, sum. sum takes a list, set, or tuple of numbers and returns the sum of all elements.

[X] Early validation of idea and implementation plan: Terraform's core is complicated enough that there are often several ways to implement something, each of which has different implications and tradeoffs. Working through a plan of attack with the team before you dive into implementation will help ensure that you're working in the right direction.

[X] Unit tests: Terraform's core is covered by hundreds of unit tests at several different layers of abstraction. Generally the best place to start is with a "Context Test". These are higher level test that interact end-to-end with most of Terraform's core. They are divided into test files for each major action (plan, apply, etc.). Getting a failing test is a great way to prove out a bug report or a new enhancement. With a context test in place, you can work on implementation and lower level unit tests. Lower level tests are largely context dependent, but the Context Tests are almost always part of core work.

[X] Documentation updates: If the core change involves anything that needs to be reflected in our documentation, you can make those changes in the same PR. The Terraform website source is in this repo and includes instructions for getting a local copy of the site up and running if you'd like to preview your changes.

[X] Well-formed Code: Do your best to follow existing conventions you see in the codebase, and ensure your code is formatted with go fmt. (The Travis CI build will fail if go fmt has not been run on incoming code.) The PR reviewers can help out on this front, and may provide comments with suggestions on how to improve the code.

[noahmercado]

@alisdair turns out I'm not able to assign you the PR but figured tagging in a comment would suffice :)

[alisdair]

Thanks! Closing and reopening to hopefully trigger a CI build 🤞

[codecov]

Codecov Report

Merging #24666 into master will increase coverage by 0.00%.
The diff coverage is 90.90%.

Impacted Files	Coverage Δ
lang/funcs/collection.go	78.41% <90.47%> (+0.98%)	⬆️
lang/functions.go	92.62% <100.00%> (+0.06%)	⬆️
terraform/node_resource_plan.go	91.59% <0.00%> (-1.69%)	⬇️
dag/marshal.go	52.27% <0.00%> (-1.14%)	⬇️
terraform/evaluate.go	41.70% <0.00%> (-0.66%)	⬇️
states/statefile/version4.go	55.32% <0.00%> (+0.25%)	⬆️
backend/remote/backend_common.go	55.63% <0.00%> (+0.70%)	⬆️

[alisdair]

This is great, thank you for your contribution! 🎉

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.