Allow configuring root path like argocd

[han-steve]

Checklist

• I've searched the issue queue to verify this is not a duplicate feature request.
• I've pasted the output of kargo version, if applicable.
• I've pasted logs, if applicable.

Proposed Feature

Add a new parameter, rootPath, to allow serving the webapp and api from a different url path. For ArgoCD, we can use the following args to serve the web app at the /argo endpoint:

args:                        
     - /usr/local/bin/argocd-server                                       
     - --insecure                                                   
     - --rootpath=/argo  

Motivation

We currently use the same base url for all of our apps that are behind our reverse proxy. For example, argo cd would be hosted at https://base-url/argo, and kargo would be at https://base-url/kargo. However, without the ability to specify a base path for the backend server, the kargo web app would make request to https://base-url/assets, which wouldn't work with our reverse proxy. We need the request to go to https://base-url/kargo/assets.

Suggested Implementation

Since we serve the frontend and backend from the same go server, we can use argo cd's approach here:
https://github.com/argoproj/argo-cd/blob/c09e6fa6ad2f48017e2d66e65a679226414e6653/server/server.go#L1122

(is there a reason we bundle everything (controller, backend, frontend etc) in the same docker container?)

[danielloader]

Could your ingress/httpRoute objects not do a path rewrite to strip the prefix?

[han-steve]

Yes, we can. But the problem is that subsequent requests wouldn’t have the /kargo route, so we don’t know to route them to kargo

[punkwalker]

Wow, I have been trying to hack it for last 2 days and finally gave up. We need this option to host kargo and other apps behind same proxy. I guess we can also allow something like --basehref for UI.
https://github.com/argoproj/argo-cd/blob/55f8a434d06ee335b195783a0a83c6a7c8dd3078/docs/operator-manual/ingress.md?plain=1#L829