refactor: update http sample (GoogleCloudPlatform#7193)

## Description

Update http sample to use declarative function signature.

## Checklist
- [x] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md)
- [x] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#readme-file)
- [x] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [x] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [x] Please **merge** this PR for me once it is approved.
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/.github/CODEOWNERS) with the codeowners for this sample

Author: KaylaNguyen

functions/helloworld/main.py

@@ -17,6 +17,7 @@
 # [START functions_helloworld_http]
 # [START functions_http_content]
 from flask import escape
+import functions_framework
 
 # [END functions_helloworld_http]
 # [END functions_http_content]
@@ -157,6 +158,7 @@ def hello_content(request):
 
 
 # [START functions_http_method]
+@functions_framework.http
 def hello_method(request):
     """ Responds to a GET request with "Hello world!". Forbids a PUT request.
     Args: