fix: change watch API

[belltoy]

The current WatchClient::watch function await the first response to the create request and keep the watch_id in the Watcher. This is not necessary, either a good idea for a etcd watch API.

The etcd Watch API behaves differently than other unary RPC. It doesn't send grpc-status in the trailers if something goes wrong. Instead, we must solve the result via the WatchResponse and its corresponding request.

Changes

The watch function returns a tuple (WatchResponse, Watcher, WatchStream), the first element in the returned tuple is the response to the first create watch request.

The Watcher struct doesn't hold the first watch_id inside. The Watcher::cancel function will always need a watch_id as parameter.

The returned WatchResponse to the first WatchCreateRequest includes more useful information to upstream applications. For example, if the request with a particular start_revision which may have been compacted in the server, then the client should never retry with the same start_revision.

BTW, in this PR, I still await the first response and try to solve the result of the first create request. But I prefer leaving it to the higher level watcher: belltoy#1

[belltoy]

 error: the `Err`-variant returned from this function is very large
    --> src/client.rs:156:71
     |
 156 |     fn build_endpoint(url: &str, options: &Option<ConnectOptions>) -> Result<Endpoint> {
     |                                                                       ^^^^^^^^^^^^^^^^
     |
    ::: src/error.rs:24:5
     |
 24  |     GRpcStatus(tonic::Status),
     |     ------------------------- the largest variant contains at least 176 bytes
     |
     = help: try reducing the size of `error::Error`, for example by boxing large elements or replacing it with `Box<error::Error>`
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#result_large_err
     = note: `-D clippy::result-large-err` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(clippy::result_large_err)]`

Failure in the clippy check, but didn't change the Error type. Found that warning was shows from rust 1.87.0. grpc/grpc-rust#2253

[belltoy]

use #108