Ability to pass env-variables to UI Extension

[dlhck]

Migrated from #3045

Is your feature request related to a problem? Please describe.
Sometimes there are API Keys or other variables that you need to switch between dev/staging and production.

Currently there is no configuration that would pass variables from Plugin to it's UI.

Not talking about the customField-config that passes customField-related data.

Describe the solution you'd like
A config-field in VendurePlugin-config that would pass the other-than-custom-field-data to UI-extension provided inside that plugin.

Describe alternatives you've considered
Allow creation of separate UI-components for different environments and a config-flag that would set the environment for which the Admin UI is built.

Additional context
Related Discord-thread:
https://discord.com/channels/1100672177260478564/1281528744527528018

[fcFn]

So a really hacky way for me right now is to simply put an .env file with my env variables into node_modules/@vendure/dashboard/.env via a script that is run before build or dev scripts, e.g.:

// ./package.json
...
"dev": "./inject-env.sh && concurrently --kill-others npm:dev:*",
...

# ./inject-env.sh
#!/bin/bash

# A hacky way to add our own ENV variables to the Vendure dashboard. 
source .env
VARS=$(cat <<EOF
VITE_MY_VAR="$MY_VAR"
VITE_MY_OTHER_VAR="$MY_OTHER_VAR"
EOF
)
echo "$VARS" > node_modules/@vendure/dashboard/.env

I haven't tried it yet on the cloud which doesn't save an .env file (and of course it's not versioned) and just injects the variables directly into the environment itself, but that should just mean that we effectively skip the source .env line and use the variables that your cloud provider inserted.

[LucidityDesign]

I'm doing it like this:

export default defineConfig(({ mode }) => {
	const env = loadEnv(mode, process.cwd(), ['PUBLIC_', 'PORT']);
	console.log(env);
	return {
		base: '/dashboard',
		envPrefix: 'PUBLIC_',
		define: {
			'import.meta.env.PUBLIC_AUTH_ISSUER': JSON.stringify(
				env.PUBLIC_AUTH_ISSUER,
			),
			// private envs will be undefined
			'import.meta.env.PRIVATE_AUTH_CLIENT_SECRET': JSON.stringify(
				env.PRIVATE_AUTH_CLIENT_SECRET,
			),
		},
		...