Support modifying a transaction category

through the PUT /transactions/{transactionId} method.

Restrict the PUT to changing only the category. The other existing
attributes should remain immutable.
Remove the body of the PUT response, it isn't required, and it was
returning a 204, which shouldn't have it.

This patch also extracts the CategoryName as a separate component on the
OpenAPI spec, so that it can be reused on the Transaction.

Issues #26 and #23

.github/workflows/go.yml

@@ -0,0 +1,28 @@
+name: Go
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go-version: [ '1.21', '1.22' ]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ matrix.go-version }}
+
+      - name: Build
+        run: go build -v ./...
+
+      - name: Test
+        run: go test -v ./...