[feat] extension to support variable length strings in SoA

fixed-string  = 'S' int-type length        ; fixed-size string
dict-string   = '[' '$' 'S' '#' count 1*(string-value)   ; dictionary-based string
offset-string = '[' '$' int-type ']'       ; offset-table-based variable string
highprec-spec = fixed-highprec | dict-highprec | offset-highprec
fixed-highprec= 'H' int-type length        ; fixed-size high-precision number
dict-highprec = '[' '$' 'H' '#' count 1*(highprec-value) ; dictionary-based high-prec
offset-highprec = '[' '$' int-type ']'     ; offset-table-based variable high-prec (same as string)

Author: fangq