Export endpoint types from generated endpoint modules

[OlliTietavainenVaadin]

Describe your motivation

Related issue: #2788

In order to make the development experience better in service workers, it would be useful if the generated endpoint modules contained explicit exports for the method signatures' types. This would allow typesafe development for method calls in service worker code.

Describe the solution you'd like

An endpoint method signature contains a name, 0...n input arguments, and a return type. The generated modules could export a list of TypeScript signature types that contain this information which could be safely imported by a service worker.

Pseudo-code example

//java
@BrowserCallable
@PermitAll
public class ProductEndpoint {

	@Nonnull
	public Product getProduct(int id, boolean x) {
		// Implementation details
	}
	
}

@Data
class Product() {
	private @Nonnull String name;
	private int id;
}

// ts
import { getSignatures } from 'Frontend/generated/endpoints/ProductEndpoint.js';

// Example 
type KnownSignatures = 'getProduct' | ...
type SignatureType = [ <method name>, <arguments>, <return type> ]

function getSignature(methodName : KnownSignatures) : SignatureType
// End of example


const [methodName, arguments, returnType] = getSignatures("get")
...
// Mock definitions
methodName : string
arguments : [number, boolean] // Option 1
arguments : {id: number, x: boolean} // Option 2
returnValue : Product;

Describe alternatives you've considered

Like in #2788 , the signature generation could be optional and configurable.

A longer-term solution could be transparent service worker call support in endpoints; not sure what that would look like exactly.

Additional context

Currently generated endpoint TS modules can't be imported in a service worker, as the Flux dependency breaks the TS build. Presumably, this wouldn't be an issue for exporting only the signature types.

Flux error when importing in sw.ts:

RollupError: node_modules/@vaadin/hilla-frontend/FluxConnection.js (1:7): "default" is not exported by "node_modules/atmosphere.js/lib/browser.js", imported by "node_modules/@vaadin/hilla-frontend/FluxConnection.js".
    at getRollupError (file:///app/node_modules/rollup/dist/es/shared/parseAst.js:396:41)
    at error (file:///app/node_modules/rollup/dist/es/shared/parseAst.js:392:42)
    at Module.error (file:///app/node_modules/rollup/dist/es/shared/node-entry.js:13858:16)
    at Module.traceVariable (file:///app/node_modules/rollup/dist/es/shared/node-entry.js:14306:29)
    at ModuleScope.findVariable (file:///app/node_modules/rollup/dist/es/shared/node-entry.js:11984:39)
    at ChildScope.findVariable (file:///app/node_modules/rollup/dist/es/shared/node-entry.js:7432:38)
    at ClassBodyScope.findVariable (file:///app/node_modules/rollup/dist/es/shared/node-entry.js:7432:38)
    at ChildScope.findVariable (file:///app/node_modules/rollup/dist/es/shared/node-entry.js:7432:38)
    at ChildScope.findVariable (file:///app/node_modules/rollup/dist/es/shared/node-entry.js:7432:38)
    at FunctionScope.findVariable (file:///app/node_modules/rollup/dist/es/shared/node-entry.js:7432:38)