GetBackend with functional params instead of GetBackendWithParams

[wojas]

Instead of deprecating GetBackend in favor of a new GetBackendWithParams, extend the GetBackend signature with variadic functional params. This way existing code can simply keep calling the same function, and we have even more flexibility in the options we provide in the future.

This drops the GetBackendWithParams that was added in PR #17.

Example usage:

	storage, err := simpleblob.GetBackend(
		ctx,
		"memory",
		map[string]any{
			// add key-value options here
			"foo": "example",
		},
		simpleblob.WithLogger(logr.Discard()), // replace with a real logger
	)

[ahouene]

Very good idea, just one concern I have commented in plugins.go :)