etcd support

[niyue]

Hi there,

Thanks for creating the project, and litefs seems a good fit for my use case, but internally we use etcd instead of consul, and we would like to reduce dependency as much as possible (which is why users are using sqlite in many cases). I wonder if there is any chance we add an additional abstraction for consul so that both etcd and consul can be used? Thanks.

[benbjohnson]

Yes, I'd like to support etcd & the Kubernetes Lease API. There are a few other items that are higher priority that I need to get to first (e.g. WAL support, write forwarding, & S3 support).