feat: support get private member method

[smallchimney]

What type of PR is this?

feat: A new feature

What this PR does / why we need it (en: English/zh: Chinese):

en: 1. We need to mock some method even if it's not exported, like (*sha256.digest).checksum or other method; 2. We need some helper function to migrate gomonkey.ApplyPrivateMethod.
zh: 1. 某些场景下我们需要Mock私有方法，比如(*sha256.digest).checksum；2. 我们需要一个辅助函数去实现从gomonkey.ApplyPrivateMethod的代码实现迁移。

[smallchimney]

Is there any possible to upgrade GetPrivateMethod to generic function in place?

[smallchimney]

I've tryed to resolve function type from reflect information, but saddly the type info of private methods have not been written for reflect package

[smallchimney]

btw, the branch develop described in CONTRIBUTION.md is not exist yet.

[smallchimney]

add the missing license decriptions

[ycydsxy]

Good job. But some tests failed ...

[Sychorius]

Try put generics-required code into another .go file with compile option like this:

//go:build go1.18
// +build go1.18

[smallchimney]

The CI testing is failed with Go1.13, which reflect.Pointer has not been declared, I changed to use reflect.Ptr for the compatibility

[ycydsxy]

some tests still failed

[smallchimney]

fixed, I've tried go1.13, 1.16, 1.17, 1.18, 1.19 and 1.20 on my dev env.