Add function to convert key=value slice pairs to map

[yorinasub17]

This adds a new function that helps with converting a key=value slice of strings to a map. This is useful for working with the return value of os.Environ().

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

• Added a new function collections.KeyValueStringSliceAsMap which can be used to work with a slice of key=value pairs.

[zackproser]

LGTM!

[yorinasub17]

Thanks for review! Merging in now.