infoschema: Putting all variables & configs into one view, with relevant information

## Enhancement
Currently, there are two types of "parameters"(variable, config) that control the behavior of instances (TiDB, TiKV, PD, TiFlash). They have different characteristics and different scopes.

Where to see variables?
- information_schema.global_variables and information_schema.session_variables
- information_schema.variables_info

Where to see configs?
- cluster_config

I would like to propose one table to see all variables and configs:
```
mysql> desc information_schema.tidb_params;
+--------------------+---------------------+------+------+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------------+---------------------+------+------+---------+-------+
| COMPONENT | varchar(64) | NO | | NULL | |
| TYPE | varchar(64) | NO | | NULL | |
| NAME | varchar(256) | NO | | NULL | |
| INSTANCE | varchar(64) | YES | | NULL | |
| SCOPE | varchar(64) | NO | | NULL | |
| VALUE | longtext | NO | | NULL | |
| DEFAULT_VALUE | varchar(64) | YES | | NULL | |
| MIN_VALUE | bigint(64) | YES | | NULL | |
| MAX_VALUE | bigint(64) unsigned | YES | | NULL | |
| POSSIBLE_VALUES | varchar(256) | YES | | NULL | |
| IS_CLUSTER_DYNAMIC | tinyint(1) | NO | | 0 | |
| IS_INST_DYNAMIC | tinyint(1) | NO | | 0 | |
| IS_SESS_DYNAMIC | tinyint(1) | NO | | 0 | |
+--------------------+---------------------+------+------+---------+-------+
13 rows in set (0.00 sec)
```
- COMPONT: "tidb/tikv/pd/tiflash" and all variables' type are "tidb"
- TYPE: VAR, CONFIG
- NAME: variable or config name
- INSTANCE: Endpoints of relevant instances.
- For INSTANCE variables: {endpoint}
- For other variables: *
- For config: {endpoint}
- SCOPE:
- GLOBALE: global variables
- INSTANCE: instance variables, all configs
- SESSION: session variable
- VALUE: current value
- DEFAULT_VALUE: default value
- MIN_VALUE: min value(config: null)
- MAX_VALUE: max value(config: null)
- POSSIBLE_VALUES: only for enum type variables(config: null)
- IS_CLUSTER_DYNAMIC: param is cluster-wide and can be set dynamically
- Global Variables: true
- Other: false
- IS_INST_DYNAMIC: param is instance-wide and can be set dynamically
- Instance variables: true
- Configs that can be set dynamically: true
- Other: false
- IS_SESS_DYNAMIC: param is session-wide and can be set dynamically
- Session Variables: YES
- Other: NO

With this new table, users can learn more information about the parameters of TiDB.