feat: Add Runnable.close() to close any resources associated with it

[davidmigloz]

Previously only LLMs and ChatModels had a close method to close the associated client, but it was not a standard interface. Now the close method is part of the Runnable interface, so you can close any Runnable.

For example:

final chain = promptTemplate
    .pipe(model)
    .pipe(outputParser);
// ...
chain.close();

This will call the close method of all the runnables in the chain. In this case, it won't do anything for promptTemplate and outputParser as they have no associated resources to close, but it will close the HTTP client of the model.