Added support for hashing with secret key. Android only

[brystfire08]

Usage:

let myAlgorithm = "HmacMD5"; // can be HmacSHA1, HmacSHA256, etc...
let mySecretKey = "secret"; // secret key

RNFetchBlob.fs.hashWithKey(targetPath, myAlgorithm, mySecretKey)
.then((hash) => {
  console.log(hash)
})
.catch((error) => {
  console.log(error);
});

if this is accepted hopefully someone apply it for ios too

[Traviskn]

I would prefer to avoid adding new API that only supports a single platform, unless there is something inherently platform specific about the functionality. If someone were willing to also implement iOS support then I would be more open to adding this new functionality