plugin: support logging rejected connection attempts in audit log

[lysu]

What problem does this PR solve?

fixes #14563

releated PR in plugin https://github.com/pingcap/enterprise-plugin/pull/25

What is changed and how it works?

• add new reject audit type
• pass reject reason via ctx

Check List

Tests

• Manual test (add detailed scripts or steps below)

build plugin and load it with new audit plugin 
login fail
see 

[2020/02/03 12:33:54.921 +08:00] [INFO] [logger.go:70] [ID=15807044340] [TIMESTAMP=2020/02/03 12:33:54.921 +08:00] [EVENT_CLASS=CONNECTION] [EVENT_SUBCLASS=Reject] [STATUS_CODE=0] [COST_TIME=0] [HOST=127.0.0.1] [CLIENT_IP=127.0.0.1] [USER=root] [DATABASES="[test]"] [TABLES="[]"] [SQL_TEXT=] [ROWS=0] [CLIENT_PORT=34160] [CONNECTION_ID=2] [CONNECTION_TYPE=Socket] [SERVER_ID=1] [SERVER_PORT=4000] [DURATION=0] [SERVER_OS_LOGIN_USER=robi] [OS_VERSION="Linux 5.3.18-1-MANJARO.x86_64"] [CLIENT_VERSION=] [SERVER_VERSION=v4.0.0-beta-51-gc46044f02-dirty] [AUDIT_VERSION=] [SSL_VERSION=v1.2.0] [PID=30808] [Reason="[server:1045]Access denied for user 'root'@'127.0.0.1' (using password: YES)"]

in audit log

Code changes

• Has exported function/method change

Side effects

• need cherry-pick to keep 3.0, 2.1 CI works

Related changes

• Need to cherry-pick to the release branch

Release note

• support logging rejected connection attempts in audit log.

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This change is 

[lysu]

/rebuild

[lysu]

@kolbe Hi, I tried and found maybe we'd better just add new Event Type 'Reject' to manifest, so EventType will be

• PreAuth
• Connected
• Reject
• Disconnect
• ChangeUser

maybe it's more uniform for log output.

and here I choose pass reject reason by context.Context, in mysql it often use threadlocal variable to do that, but golang doesn't have that, but context.Context maybe suitable for this? 😞 we are hard to predefine all params for plugins, pre-define a context param maybe ok?

PTAL if free 😄

[kolbe]

LGTM

[jackysp]

LGTM

[jackysp]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

cherry pick to release-3.0 failed