add standalone launch gateway

[KeeyanGhoreshi]

Describe your changes

Adds a standalone launch page when opening the app on the base url. When opening localhost:3000 the app will show a form that allows the user to input the iss/client/scopes before launching. Inputs default to the value in the env file so if you just want to connect to the regular ehr, you can launch immediately.

You can test by changing the values in the inputs and seeing that it connects to the chosen ehr.

There is some room here to connect with the client id registration task and have the dropdown menus include a list of iss/clients, and automatically loading the associated iss/client when you choose the counterpart.

Issue ticket number and Jira link

REMS-573

Checklist before requesting a review

• I have performed a self-review of my code
• Ensure the target / base branch for any feature PR is set to dev not main (the only exception to this is releases from dev and hotfix branches)

Checklist for conducting a review

• Review the code changes and make sure they all make sense and are necessary.
• Pull the PR branch locally and test by running through workflow and making sure everything works as it is supposed to.

[plarocque4]

This works great to connect to the test-ehr running locally, but I am having trouble connecting to meld.
My settings are the following:
FHIR Server Endpoint: https://gw.interop.community/REMS/data
Client ID: ed8b940e-4aaa-4209-b17d-69dfe67543b9
Scope: launch offline_access openid profile user/.

It complains about the scopes. I get the same error when I try the default scopes. The scopes I have listed here are what worked for me when I set them in the .env file and was able to connect.

[KeeyanGhoreshi]

recent commit should fix it, note that the scopes must match what meld has registered for the specific client. Also the test-ehr does not validate scopes so this error did not occur there.

[plarocque4]

Works well