fix(gateway): allow custom proxyFactory to use non-default proxyTypes

[Vincentdevreede]

This PR improves flexibility in the gateway by allowing custom proxyFactory implementations to define and use custom proxyType values without triggering an "Unsupported proxy type" error.

Changes

• Wrapped opts.proxyFactory with a fallback to defaultProxyFactory only if it returns undefined
• Skip proxyType validation when a custom proxyFactory is provided
• Default missing proxy hooks to NOOP functions

Previously, providing a custom proxyFactory and proxyType would still result in an error unless the type was listed in the internal PROXY_TYPES list. This fix ensures full control when extending proxy behavior through configuration.

// ——————————————————————————
// 1) “Normal” mode: return undefined to fall back
// ——————————————————————————
gateway({
  proxyFactory: (ctx) => {
    const { proxyType, opts, route } = ctx;

    if (proxyType === 'custom') {
      return myCustomProxy;
    }

    // returning undefined ⇒ defaultProxyFactory will be used
  }
});

// ——————————————————————————
// 2) “Disable fallback” mode: return null to opt out
// ——————————————————————————
gateway({
  proxyFactory: (ctx) => {
    const { proxyType, opts, route } = ctx;

    if (proxyType === 'custom') {
      return myCustomProxy;
    }

    // returning null ⇒ no fallback
    return null;
  }
});

[jkyberneees]

Hi @Vincentdevreede many thanks for supporting the library evolution. I definitively like the proposal and makes total sense.

Could you please fix linting issues and extend the docs to reflect your proposal? Afterwards I will proceed with merging it.