Configurable internal parameters (e.g., parallelization)

[kevinmichaelchen]

We can allow consumers of this library to configure certain parameters, such as parallelization factors, using the Functional Options pattern:

type config struct {
	parallelization int
}

func newConfig() config {
	return config{
		parallelization: 10,
	}
}

type BetweenOption func(*config)

// WithParallelization configures the level of parallelization to use for
// concurrent Distance Matrix API requests.
func WithParallelization(parallelization int) BetweenOption {
	return func(c *config) {
		c.parallelization = parallelization
	}
}

func BetweenPoints(
	ctx context.Context,
	client *maps.Client,
	in distance.BetweenPointsInput,
	options ...BetweenOption,
) (*distance.MatrixResponse, error) {}