Allow promotes to include named links (#65)

The text and URL of the link come from a provided template file.

For now, the only variable that can be passed to the template is
docker-image-sha256-63.

(We have designed this specifically so that database migration
promotions can link to GCP logs for a k8s job that ran `flyway info`.)

Author: glasser

__tests__/__fixtures__/templates/templates.yaml

@@ -0,0 +1,8 @@
+link-one:
+  text:
+    - literal: 'Link to '
+    - variable: foo
+  url:
+    - literal: 'https://some-site.example/logs/query?foo='
+    - variable: 'foo'
+    - literal: '&bar=yay'

__tests__/templates.test.ts

@@ -0,0 +1,27 @@
+import { join } from 'path';
+import { readLinkTemplateMapFile, renderLinkTemplate } from '../src/templates';
+
+function fixtureFilename(filename: string): string {
+  return join(__dirname, '__fixtures__', 'templates', filename);
+}
+
+describe('templates', () => {
+  it('renders a template', async () => {
+    const templateMap = await readLinkTemplateMapFile(
+      fixtureFilename('templates.yaml'),
+    );
+    expect(
+      renderLinkTemplate(
+        templateMap,
+        'link-one',
+        new Map([
+          ['foo', 'hooray'],
+          ['bla', 'another'],
+        ]),
+      ),
+    ).toStrictEqual({
+      text: 'Link to hooray',
+      url: 'https://some-site.example/logs/query?foo=hooray&bar=yay',
+    });
+  });
+});

action.yml

@@ -42,6 +42,9 @@ inputs:
   generate-promoted-commits-markdown:
     description: 'Generates the promoted-commits-markdown output'
     default: 'false'
+  
+  link-template-file:
+    description: 'If provided, a path to a YAML file mapping from template names to link templates. A template is a `text` and an `url`, each of which is a list of objects, each of which is of the form `{literal: "literal text"}` or `{variable: "variable-name"}.'
 
 outputs:
   suggested-promotion-branch-name: