Route explain tab plan to the proper Keyspace

[frouioui]

Description

This pull request introduces FindKeyspace to the VSchema interface, allowing us to route explain queries to the proper keyspace regardless if the target is a MySQL table or a Vindex table.

Based on the issue reported in #10019, running a query that describes a Vindex led to a panic in VTGate. When running the local example to have a sharded cluster, the queries that follow used to crash:

mysql> use customer;
mysql> desc hash;
ERROR 2013 (HY000): Lost connection to MySQL server during query
No connection. Trying to reconnect...
Connection id:    8
Current database: customer

ERROR 2013 (HY000): Lost connection to MySQL server during query
No connection. Trying to reconnect...
Connection id:    9
Current database: customer

ERROR 2013 (HY000): Lost connection to MySQL server during query
mysql> 

With the patch proposed in this pull request, the same queries produce the output that follows.

mysql> use customer;
mysql> desc hash;
ERROR 1146 (42S02): target: customer.80-.primary: vttablet: rpc error: code = NotFound desc = Table 'vt_customer.hash' doesn't exist (errno 1146) (sqlstate 42S02) (CallerID: userData1): Sql: "explain `hash`", BindVars: {}

The MySQL error is expected as there is no hash table in this keyspace. Describe/explain are used to obtain information about table structure or query execution plans, not indexes.

Related Issue(s)

• Fixes Bug Report: planbuilder vtgate panic on DESC of hash vindex #10019

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required