Set default user agent in proxy client

[welteki]

Description

Set a default user agent for the http proxy client, openfaas-ce-gateway/<version> if the proxied request does not have the User-Agent header.

Motivation and Context

• I have raised an issue to propose this change (required)
• My issue has received approval from the maintainers or lead with the design/approved label

The gateway used to inject the default golang user agent, Go-http-client/1.1, into requests. This change ensures the default user agent is set correctly.

How Has This Been Tested?

Verified the gateway user agent header is injected if a request does not contain a user agent header.

curl -i "http://127.0.0.1:8001/function/env" -H "User-Agent:"

TTP/1.1 200 OK
Content-Length: 994
Content-Type: text/plain; charset=utf-8
Date: Tue, 06 Aug 2024 13:56:26 GMT
X-Call-Id: 38929e79-5e13-4369-a9d1-93a71101a058
X-Duration-Seconds: 0.000604
X-Served-By: openfaas-community/
X-Start-Time: 1722952586671966988

...
Http_User_Agent=openfaas-ce-gateway/dev
...

Verified the user agent from the original request is forwarded.

curl -i "http://127.0.0.1:8001/function/env"      
HTTP/1.1 200 OK
Content-Length: 982
Content-Type: text/plain; charset=utf-8
Date: Tue, 06 Aug 2024 13:55:37 GMT
X-Call-Id: 5719114d-1032-41ae-87ac-813eefcc4019
X-Duration-Seconds: 0.000715
X-Served-By: openfaas-community/
X-Start-Time: 1722952537702282995

...
Http_User_Agent=curl/7.81.0
...

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.