expression: implement vectorized for builtinCastStringAsRealSig

[eggeek]

What problem does this PR solve?

1. Add vectorized expression: builtinCastStringAsRealSig
2. Add the corresponding test case
3. Add the corresponding generator: realStringGener

What is changed and how it works?

goos: darwin
goarch: amd64
pkg: github.com/pingcap/tidb/expression
BenchmarkVectorizedBuiltinCastFunc/builtinCastStringAsRealSig-VecBuiltinFunc-8             10000            102142 ns/op               0 B/op          0 allocs/op
BenchmarkVectorizedBuiltinCastFunc/builtinCastStringAsRealSig-NonVecBuiltinFunc-8           8454            132102 ns/op               0 B/op          0 allocs/op
PASS
ok      github.com/pingcap/tidb/expression      2.785s

Check List

Tests

• Unit test

Code changes

• Has exported function/method change
• Has interface methods change

Side effects

• No

Related changes

• No

Release note

• No

[codecov]

Codecov Report

Merging #13952 into master will decrease coverage by 0.017%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             master     #13952        +/-   ##
================================================
- Coverage   80.4459%   80.4288%   -0.0171%     
================================================
  Files           480        480                
  Lines        121627     121255       -372     
================================================
- Hits          97844      97524       -320     
+ Misses        16093      16035        -58     
- Partials       7690       7696         +6

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