Enforce import order using gci

Add linting to the pre-merge checks to enforce Go import groups:
- stdlib
- 3rd-party / non-Pebble imports
- Pebble imports

.github/.golangci.yml

@@ -0,0 +1,10 @@
+linters:
+  disable-all: true
+  enable:
+    - gci
+linters-settings:
+  gci:
+    sections:
+      - standard
+      - default
+      - Prefix(github.com/canonical/pebble)

.github/workflows/lint.yml

@@ -0,0 +1,20 @@
+name: Lint
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - uses: actions/setup-go@v4
+        with:
+          go-mod-file: 'go.mod'
+          cache: false
+
+      - name: golangci-lint
+        uses: golangci/golangci-lint-action@v3
+        with:
+          version: latest
+          args: '-c .github/.golangci.yml'
+          skip-cache: true