feat: add multiple config files support to globals

[katcipis]

This PR allows globals to be defined on multiple files. We still lack suppor to export_as_locals and generate_hcl.

Example:

#!/bin/bash

set -o errexit
set -o nounset

basedir=$(mktemp -d)

cd "${basedir}"

cat > terramate.tm.hcl <<- EOM
terramate {
  config {
  }
}
EOM

mkdir -p stacks/stack-1
mkdir -p stacks/stack-2

terramate stacks init stacks/stack-1
terramate stacks init stacks/stack-2

echo "terramate list"
echo

terramate stacks list

echo

cat > stacks/globals1.tm.hcl <<- EOM
globals {
  testing = "test value"
}
EOM

cat > stacks/globals2.tm <<- EOM
globals {
  object = {
    name = "hello"
    data = "world"
  }
}
EOM

echo "terramate stacks globals"

terramate stacks globals

[codecov-commenter]

Codecov Report

Merging #221 (deb6890) into main (d68152f) will increase coverage by 0.14%.
The diff coverage is 90.99%.

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
+ Coverage   63.57%   63.71%   +0.14%     
==========================================
  Files          29       29              
  Lines        5403     5438      +35     
==========================================
+ Hits         3435     3465      +30     
- Misses       1784     1789       +5     
  Partials      184      184              

Flag	Coverage Δ
tests	63.71% <90.99%> (+0.14%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
globals.go	84.04% <89.39%> (-1.23%)	⬇️
hcl/hcl.go	81.53% <93.33%> (+0.50%)	⬆️

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[i4ki]

LGTM