Refactoring proxy lifetime to only shutdown when proxy is out-of-date. (#839)

## Summary of the Pull Request

_What is this about?_
We'd like to refactor the proxy lifecycle to only delete when the proxy is out-of-date - i.e. when the proxy is older than 7 days or a mismatched version. I've changed two files, proxy.py and timer_daily\init.py to check for the version and timestamp before stopping a live proxy. 

## PR Checklist
* [ ] Applies to work item: #xxx
* [ ] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/onefuzz) and sign the CLI.
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

## Info on Pull Request

_What does this include?_
Changes to two files: 
proxy.py: 
- get_or_create() edited to check if timestamp is >7 days.
- Created is_outdated() to check version and timestamp for out-of-date proxy. 
timer_daily/init.py
- Proxy check now includes is_outdated() before determining if a proxy should be shutdown. 

## Validation Steps Performed
Deploying test instance to determine if proxy lives past a single day.

src/api-service/__app__/proxy/__init__.py

@@ -79,6 +79,8 @@ def post(req: func.HttpRequest) -> func.HttpResponse:
 
     proxy = Proxy.get_or_create(scaleset.region)
     if proxy:
+        forward.proxy_id = proxy.proxy_id
+        forward.save()
         proxy.save_proxy_config()
     return ok(get_result(forward, proxy))