Send emails to the team on Nightly CI Failure (#14)

Reuses the Admin SDK's Send Email action along with Firebase's MailGun credentials to notify the team if the nightly CI fails.

.github/actions/send-email/README.md

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+# Send Email GitHub Action
+
+This is a minimalistic GitHub Action for sending emails using the Mailgun API.
+Specify the Mailgun API key along with the Mailgun domain and message to
+be sent.
+
+## Inputs
+
+### `api-key`
+
+**Required** Mailgun API key.
+
+### `domain`
+
+**Required** Mailgun domain name.
+
+### `from`
+
+**Required** Sender's email address. Ex: 'Hello User <[email protected]>' (defaults to 'user@{domain}`).
+
+### `to`
+
+**Required** Recipient's email address. You can use commas to separate multiple recipients.
+
+### `cc`
+
+Email addresses to Cc.
+
+### `subject`
+
+**Required** Message subject.
+
+### `text`
+
+Text body of the message.
+
+### `html`
+
+HTML body of the message.
+
+## Example usage
+
+```
+- name: Send Email
+  uses: firebase/firebase-sdk-nextjs-tests/.github/actions/send-email
+  with:
+    api-key: ${{ secrets.MAILGUN_API_KEY }}
+    domain: ${{ secrets.MAILGUN_DOMAIN }}
+    from: 'User <[email protected]>'
+    html: '<h1>Testing some Mailgun awesomness!</h1>'
+    to: '[email protected]'
+```
+
+## Implementation
+
+This Action uses the `mailgun.js` NPM package to send Emails.
+
+When making a code change remember to run `npm run pack` to rebuild the
+`dist/index.js` file which is the executable of this Action.

.github/actions/send-email/action.yml

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+# Copyright 2024 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: 'Send email Action'
+description: 'Send emails using Mailgun from GitHub Actions workflows.'
+inputs:
+  api-key:
+    description: Mailgun API key.
+    required: true
+  domain:
+    description: Mailgun domain name.
+    required: true
+  from:
+    description: Senders name and email address.
+    required: true
+  to:
+    description: Recipient's email address. You can use commas to separate multiple recipients.
+    required: true
+  cc:
+    description: Email addresses to Cc.
+    required: false
+  subject:
+    description: Message subject.
+    required: true
+  text:
+    description: Text body of the message.
+    required: false
+  html:
+    description: HTML body of the message.
+    required: false
+runs:
+  using: 'node20'
+  main: 'dist/index.js'