fix: missing i8 type

[junhaideng]

Motivation

pilota build failed

struct Number {
    1: optional i8 a = 0; 
}

 thread 'main' panicked at 'unexpected literal Int(0) with ty I8', .../rsproxy.cn-8f6827c7555bfaf8/pilota-build-0.7.7/src/middle/context.rs:678:18

Solution

code gen i8 type

[CLAassistant]

All committers have signed the CLA.

[PureWhiteWu]

Seems that there's no i8 type in the thrift types doc: https://thrift.apache.org/docs/types