[SQL] Support MySQL IF&IFNULL Function

[gongna-au]

What this PR does:

Which issue(s) this PR fixes:

Fixes #493

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Base: 35.72% // Head: 35.49% // Decreases project coverage by -0.22% ⚠️

Coverage data is based on head (0e4aa9c) compared to base (d47a067).
Patch coverage: 57.14% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #588      +/-   ##
==========================================
- Coverage   35.72%   35.49%   -0.23%     
==========================================
  Files         185      188       +3     
  Lines       21199    21491     +292     
==========================================
+ Hits         7573     7628      +55     
- Misses      12753    12979     +226     
- Partials      873      884      +11     

Impacted Files	Coverage Δ
pkg/runtime/function/if.go	52.00% <52.00%> (ø)
pkg/runtime/function/if_null.go	64.70% <64.70%> (ø)
pkg/security/tenant.go	63.63% <0.00%> (-7.16%)	⬇️
pkg/runtime/ast/model.go	33.33% <0.00%> (-5.32%)	⬇️
pkg/runtime/ast/ast.go	72.66% <0.00%> (-1.11%)	⬇️
pkg/boot/discovery.go	48.03% <0.00%> (-0.89%)	⬇️
pkg/executor/redirect.go	4.08% <0.00%> (-0.85%)	⬇️
pkg/config/misc.go	0.00% <0.00%> (ø)
pkg/config/model.go	34.01% <0.00%> (ø)
pkg/mysql/server.go	0.00% <0.00%> (ø)
... and 4 more

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[jjeffcaii]

pls fix

[jjeffcaii]

pls fix