Move internal grpc client to test crate

[MichaelScofield]

What type of enhancement is this?

Refactor

What does the enhancement do?

For historic reason, there's an internal grpc client in crate "client" called "Database", for talking to Frontend's grpc service. Now it's mainly used in two places: one is in the cli data export tool, and the other is in tests. We can move the grpc client to test crate to make our codes more clear. And in the mean time, use other client (like mysql client, or simply http API) in our cli data export tool, to replace it.

Implementation challenges

No response

[tisonkun]

This is the related struct:

greptimedb/src/client/src/database.rs

Lines 42 to 58 in e410192

	#[derive(Clone, Debug, Default)]
	pub struct Database {
	// The "catalog" and "schema" to be used in processing the requests at the server side.
	// They are the "hint" or "context", just like how the "database" in "USE" statement is treated in MySQL.
	// They will be carried in the request header.
	catalog: String,
	schema: String,
	// The dbname follows naming rule as out mysql, postgres and http
	// protocol. The server treat dbname in priority of catalog/schema.
	dbname: String,
	// The time zone indicates the time zone where the user is located.
	// Some queries need to be aware of the user's time zone to perform some specific actions.
	timezone: String,
	client: Client,
	ctx: FlightContext,
	}