plugin: support flush tidb plugin {name} in tidb-server

[lysu]

What problem does this PR solve?

refer pingcap/parser#209 and prepose to #9136

What is changed and how it works?

impl was insprited by #9271

• make plugin can choosen be flushable
• support flush tidb plugin {name} command
• plugin framework help plugins watch etcd and support notify other tidb nodes about flush event
• temp remove reload logic which is too complex but use limited for current stage.

use 2 phase init, plugin can Init before domain init and start watch loops after domain be ready.

Check List

Tests

• Manual test (add detailed scripts or steps below)

script: (WIP)

Code changes

• Has exported function/method change

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

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This change is 

[lysu]

/run-all-tests

[lysu]

/run-all-tests

[tiancaiamao]

LGTM

[lysu]

/run-all-tests

[jackysp]

LGTM

[codecov-io]

Codecov Report

Merging #9320 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #9320      +/-   ##
=========================================
- Coverage    67.2%   67.2%   -0.01%     
=========================================
  Files         371     371              
  Lines       77572   77574       +2     
=========================================
+ Hits        52132   52133       +1     
  Misses      20783   20783              
- Partials     4657    4658       +1

Impacted Files	Coverage Δ
plugin/spi.go	100% <ø> (ø)	⬆️
session/session.go	72.48% <0%> (-0.16%)	⬇️
plugin/plugin.go	0% <0%> (ø)	⬆️
executor/simple.go	67.51% <0%> (+1.12%)	⬆️
executor/join.go	77.86% <0%> (-0.53%)	⬇️
executor/executor.go	67.03% <0%> (+0.13%)	⬆️
infoschema/infoschema.go	77.63% <0%> (+1.31%)	⬆️

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