Add easy cache invalidation helper to defineCachedFunction/defineCachedHandler

[schplitt]

Describe the feature

When using defineCachedFunction or defineCachedHandler in Nitro, manually invalidating cache entries is complex and error-prone. The cache key depends on the function group, name, and often function parameters, requiring users to reconstruct the cache key string for invalidation:

nitro:functions:getAccessToken:default.json

and call:

await useStorage('cache').removeItem('nitro:functions:getAccessToken:default.json');

This process is not ergonomic, especially in larger projects or when multiple cached functions exist with dynamic keys.

Proposal:
Add a built-in cache invalidation method, such as .invalidateCache(), to the function returned by defineCachedFunction/defineCachedHandler. This helper would automatically generate the correct cache key (using the provided arguments) and invalidate it, making cache management simpler for users.

Example usage:

const getAccessToken = defineCachedFunction(/* ...  */);
await getAccessToken.invalidateCache();

This reduces developer error, avoids code duplication, and improves cache ergonomics in user projects.

Additional information

• Would you be willing to help implement this feature?