*: integrate plugin framework with TiDB

[lysu]

What problem does this PR solve?

following up #8788, integration plugin framework into TiDB

What is changed and how it works?

• add plugin_dir and plugin_load config
• make loadCommonGlobalVarsSQL be tweakable before boostrap
• load plugin before boostrap

Check List

Tests

• manual test

1. checkout https://github.com/lysu/slow_log_table
2. pluginpkg slow_log_table
3. load with tidb
4. check slow log table exists

Code changes

• change Impl

Side effects

• N/A

Related changes

• N/A

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This change is 

[codecov-io]

Codecov Report

Merging #9006 into master will decrease coverage by 0.03%.
The diff coverage is 29.78%.

@@            Coverage Diff             @@
##           master    #9006      +/-   ##
==========================================
- Coverage   67.28%   67.25%   -0.04%     
==========================================
  Files         371      371              
  Lines       75930    75972      +42     
==========================================
+ Hits        51092    51094       +2     
- Misses      20332    20367      +35     
- Partials     4506     4511       +5

Impacted Files	Coverage Δ
config/config.go	14.54% <ø> (ø)	⬆️
sessionctx/variable/sysvar.go	100% <ø> (ø)	⬆️
executor/show.go	41.21% <0%> (-0.26%)	⬇️
infoschema/perfschema/tables.go	60% <0%> (-18.95%)	⬇️
planner/core/planbuilder.go	50.12% <0%> (ø)	⬆️
sessionctx/variable/varsutil.go	25.46% <0%> (-0.33%)	⬇️
domain/topn_slow_query.go	95.69% <0%> (-2.11%)	⬇️
session/session.go	72.35% <38.88%> (-0.67%)	⬇️
domain/domain.go	35.7% <60%> (-0.02%)	⬇️
infoschema/infoschema.go	78.91% <66.66%> (-0.67%)	⬇️
... and 6 more

---

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[lysu]

/run-all-tests

[lysu]

@tiancaiamao PTAL it also contain a part of #8981

[lysu]

/run-all-tests

[jackysp]

LGTM

[jackysp]

@tennix PTAL if it will affect the docker image in k8s.

[tiancaiamao]

LGTM