Consolidate fetching of MySQL server info

[timvaillancourt]

Description

This PR consolidates the many queries and functions that fetch MySQL server runtime configuration into a single struct and function: mysql.ServerInfo and mysql.GetServerInfo(...) (*ServerInfo, error) respectively. This reduces the server-info-related MySQL queries from 6 -> 2

Now server information for both the applier and inspector are stored in the migration context as single structs

Notes:

• To handle cases where @@global.port and @@global.extra_port are NULL a gosql.NullInt64 was used when cleaning up base.ValidateConnection(...)
  • This required Port from mysql.InstanceKey to become int64 as there is no gosql.NullInt
• Removed duplicate base.ValidateConnection(...) on the applier's this.singletonDB database handle as it is the same host as the already-validated this.db handle
  • This resolves these confusing/duplicate log lines:

    2022-12-10 04:20:29 INFO applier connection validated on primary:3306
    2022-12-10 04:20:29 INFO applier connection validated on primary:3306

• The Applier initiated on %+v, version %+v log-lines now include the @@global.version_comment (in brackets) for more info
• Unit test added to base.ValidateConnection(...) now that it takes in serverInfo *mysql.ServerInfo vs a harder-to-test db *sql.DB
• Moved migrationContext.RequiresBinlogFormatChange(...) to inspector because it's never called outside of inspector
  • Added unit test
• Remove unused .validateAndReadTimeZone() func from applier

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[meiji163]

HI @timvaillancourt 👋 do you still want to add this one to 1.1.7 or should we push it out? I'm planning to cut a release this week or next