refactor: change value of meta-service key DatabaseNameIdent from DatabaseId to Id<DatabaseId> (#16299)

* refactor: make Id more generic so that it can include any kind of Id type

This will be useful to make `Id<T>` a wrapper type for T to impl
`prost::Message` for T.

* refactor: change value of meta-service key `DatabaseNameIdent` from `DatabaseId` to `Id<DatabaseId>`

`Id<DatabaseId>` is a `prost::Message` and can be used directly as a
meta-service value thus it unifies get operation to meta-service.
For example, `KVPbApi::get_db()` now can be used to get a u64 id value,
instead of using `get_u64_value()`.

Author: drmingdrmer

Cargo.lock

@@ -222,7 +222,7 @@ impl<KV: kvapi::KVApi<Error = MetaError>> BackgroundApi for KV {
 
             let job_id = deserialize_u64(&v.data)?;
 
-            let req = BackgroundJobIdIdent::new(&req.tenant, job_id.0);
+            let req = BackgroundJobIdIdent::new(&req.tenant, *job_id);
             let seq_info = self.get_pb(&req).await?;
 
             // filter none and get the task info