feat: Support MySQL math function PI & TRUNCATE

[PangXing]

What this PR does:
Support MySQL math function PI & TRUNCATE

Which issue(s) this PR fixes:

Fixes #455

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Base: 36.10% // Head: 35.73% // Decreases project coverage by -0.37% ⚠️

Coverage data is based on head (4219fa2) compared to base (66321cf).
Patch coverage: 50.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
- Coverage   36.10%   35.73%   -0.38%     
==========================================
  Files         147      151       +4     
  Lines       21089    21364     +275     
==========================================
+ Hits         7615     7635      +20     
- Misses      12643    12912     +269     
+ Partials      831      817      -14     

Impacted Files	Coverage Δ
pkg/runtime/function2/truncate.go	44.00% <44.00%> (ø)
pkg/runtime/function2/pi.go	100.00% <100.00%> (ø)
pkg/boot/discovery_watch.go	3.09% <0.00%> (-21.65%)	⬇️
pkg/boot/boot.go	45.00% <0.00%> (-6.88%)	⬇️
pkg/boot/discovery.go	28.22% <0.00%> (-5.84%)	⬇️
pkg/runtime/ast/show.go	29.55% <0.00%> (-1.12%)	⬇️
pkg/dataset/priority_queue.go	64.35% <0.00%> (-1.00%)	⬇️
pkg/runtime/ast/ast.go	74.37% <0.00%> (-0.12%)	⬇️
pkg/executor/redirect.go	4.91% <0.00%> (-0.03%)	⬇️
pkg/config/tenant.go	0.00% <0.00%> (ø)
... and 3 more

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