Add the trimspace() interpolation function

[ColinHebert]

This function helps when interpolating string from UNIX files.

For example

resource "docker_container" "test" {
  name         = "test"
  image        = "${docker_image.ubuntu.latest}"
  env          = [
    "PASSWORD=${trimspace(file("files/secrets/password"))}",
  ]
}

[radeksimko]

It may not be obvious from the name that we're only trimming spaces.
Without reading the docs, one may expect trim(string, cutset) being mapped to strings.Trim().

Would you mind changing the name to trimspace()?

Otherwise this PR is looking good.

[ColinHebert]

Updated

[ghost]

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