Support async operation handler resolver

[mdmower-csnw]

• Let users define operationHandlers.resolver as a synchronous or asynchronous function that returns a request handler
• Make installOperationHandlers an asynchronous function that awaits a resolver promise (automatically wraps resolver with promise if needed)
• Update operation handlers middleware to handle an async installOperationHandlers.

This is similar to the approach mentioned in #660 but makes it possible to use await import() in the resolver so that a new request handler doesn't have to be created as a wrapper. Then, the default resolver requires very little modification. I was able to replace

express-openapi-validator/src/resolvers.ts

Lines 33 to 38 in 4e8bc84

	const modulePath = path.join(handlersPath, baseName);
	if (!tmpModules[modulePath]) {
	tmpModules[modulePath] = require(modulePath);
	}
	const handler = tmpModules[modulePath][oId] || tmpModules[modulePath].default[oId] || tmpModules[modulePath].default;

with

// import {pathToFileURL} from 'node:url';

const modulePath = path.join(handlersPath, baseName);
const moduleUrl = `${pathToFileURL(modulePath).toString()}.js`;
const importedModule = (await import(moduleUrl)) as Record<string, RequestHandler | undefined>;

const handler = importedModule[oId];

and otherwise use the default resolver as-is.