infoschema: Migrate the infoschema's retrieving data logic for sys table to executor

[reafans]

Feature Request

Is your feature request related to a problem? Please describe:

The infoschema pkg in TiDB is a SQL Meta-information module, In addition, all operations for Information Schema would access here. However, it contains too much logic for retrieving data like the dataForTableXXX() func, and these operations should have be put in the executor pkg.
relateds to #14704

Describe the feature you'd like:

We need to migrate these retrieving data logic to the executor pkg so that infoshema only contains schema meta information.
There are some other func of retrieving data in infoschema/tables.go ：

• dataForTables()
• dataForIndexes()
• dataForColumns()
• dataForStatistics()
• dataForCharacterSets()
• dataForCollations()
• dataForSessionVar()
• dataForTableConstraints()
• dataForPseudoProfiling()
• dataForPartitions()
• dataForKeyColumnUsage()
• dataForUserPrivileges()
• dataForEngines()
• dataForCollationCharacterSetApplicability()
• dataForProcesslist()
• dataForTiDBHotRegions()
• dataForTiKVStoreStatus()

...

migrate these logic and related test to executor/infoschema_reader.go , you can refer to the migration of dataForSchemata() and dataForSchemata() in #14704

Teachability, Documentation, Adoption, Migration Strategy:

Any questions for information_schema could you refer to MySQL 8.0 doc (information_schema)

[sre-bot]

Dear contributor, the first Performance challenge program has been closed. But you can still contribute to this issue and we appreciate your contribution.

[sre-bot]

Dear contributor, the first Performance challenge program has been closed. But you can still contribute to this issue and we appreciate your contribution.

[sre-bot]

Dear contributor, the first Performance challenge program has been closed. But you can still contribute to this issue and we appreciate your contribution.