Add examples about how to use all etcd operations

[viing937]

Is your feature request related to a problem? Please describe.
I want to use ETCD's ttl, and I noticed that there are some lease methods (LeaseGrant and so on) exposed from client, but I can't figure out how to use them.

Describe the solution you'd like
Add some usage examples in README.md.

Thank you for your creating the great client again.

[viing937]

I found how to set a key with ttl.

EtcdClient client = new EtcdClient("127.0.0.1", 2379);

long leaseId = client.LeaseGrant(new LeaseGrantRequest
{
    TTL = ttl
}).ID;

client.Put(new PutRequest
{
    Key = ByteString.CopyFromUtf8(key),
    Value = ByteString.CopyFromUtf8(val),
    Lease = leaseId
});

And this can keep lease alive, but I am not sure what are LeaseKeepAliveHandler and CancellationToken used for.

client.LeaseKeepAlive(new LeaseKeepAliveRequest
{
    ID = leaseId
}, LeaseKeepAliveHandler, new CancellationToken());

private static void LeaseKeepAliveHandler(LeaseKeepAliveResponse rep)
{
    Console.WriteLine($"Lease {rep.ID} keep alive");
}

[shubhamranjan]

LeaseKeepAliveHandler : The method which contains instructions to be executed on each LeaseKeepAlive response.

CancellationToken: Propagates notification that operations should be canceled.

https://docs.microsoft.com/en-us/dotnet/api/system.threading.cancellationtoken?view=netframework-4.8

[shubhamranjan]

I am working on restructuring the lib's code a bit without hurting method signatures. I will try to update documentation as soon as I can.

[viing937]

Actually I do not need to execute anything on LeaseKeepAlive response, an overload of LeaseKeepAlive without LeaseKeepAliveHandler should be better.
Also, CancellationToken should be optional as the same reason.

[shubhamranjan]

Noted. For now, you can use the overload which takes an array of Handlers and you can pass an empty array.

[spiderpoison]

I need Doc about Lease too, thanks very much

[anythink-wx]

LeaseGrant and LeaseKeepAlive doc ?