Long-term solution[Break change] on improve loadProtoJSON

[summer-ji-eng]

To speed up the loadProtoJSON and addressed #1196, we have temperately solution use hashobject(JSON.stringify(json)).

Because hashobject is work for both nodejs and browser( see package.json).

For long-term ultimate goal is to reduce the dependency(i.e remove hashobject). To get rid of hashobject cause breaking change. Because crypto is Node.js specific module. And to use in browser, we need window.crypto.subtle. But the digest is asynchronies function return promise.

Example in
https://github.com/googleapis/google-auth-library-nodejs/blob/main/src/crypto/crypto.ts#L76-L81

Draft implementation PR in gax:
0ce1a4c