feat(smithy-client): add handler cache

[kuhe]

aws/aws-sdk-js-v3#6423

This adds a cache for the handler functions resolved from calls to

command.resolveMiddleware(client.middlewareStack, client.config, (request) options)

when options is undefined. This occurs in the request path and can take 1-2ms, but is redundant for any client/command pair in most cases.

Despite being cached, the handler functions has a reference to the client config, so any changes there will still be visible to the middleware within the resolved handler. Modifying a client's middlewareStack invalidates the cache.

The cache is opt-in and also unique per client, so users retain control that way.

[kuhe]

putting this into draft while I reevaluate whether this change is worth it or not

update: the main issue was how to detect when to invalidate the cache. Rather than solving that problem, which may have made things worse for the problem the cache is solving, I've made the use of it configurable and opt-in.