Fast authentication for `caching_sha2_password`

[dveeden]

Enhancement

The caching_sha2_password authentication method supports a "fast authentication" (cached) and a "full authentication". TiDB only supports "full authentication". This issue is to add support for "fast authentication".

tidb/pkg/server/conn.go

Lines 702 to 704 in c82ba4e

	// Currently we always send a "FastAuthFail" as the cached part of the protocol isn't implemented yet.
	// This triggers the client to send the full response.
	err := cc.writePacket([]byte{0, 0, 0, 0, shaCommand, fastAuthFail})

https://dev.mysql.com/doc/dev/mysql-server/latest/page_caching_sha2_authentication_exchanges.html

Previous work: pingcap/parser#1328

Related:

• default_authentication_plugin is not compatable with MySQL's #54138 (comment)
• Support for caching_sha2_password #9411

[dveeden]

Note for pingcap/parser#1328 that pingcap/parser has been merged into pingcap/tidb.