Add file bucket encryption using fileKey

[cbaker6]

Adds fileKey encryption similar to original Parse. This is implemented for GridFSBucketAdapter and the FSFilesAdapter. This uses AES256-GCM so it can detect if files are tampered with.

To use, simply use the env var PARSE_SERVER_FILE_KEY, pass in --fileKey in the CL, or initialize ParseServer with fileKey="Your file encryptionKey". An example using GridFSBucketAdapter :

const api = new ParseServer({
  databaseURI: databaseUri || 'mongodb://localhost:27017/dev',
  cloud: process.env.PARSE_SERVER_CLOUD || __dirname + '/cloud/main.js',
  appId: process.env.PARSE_SERVER_APPLICATION_ID || 'myAppId',
  masterKey: process.env.PARSE_SERVER_MASTER_KEY || '', 
  fileKey: process.env.PARSE_SERVER_FILE_KEY, //Add your file key here. Keep it secret
  ...
});

An example using FileSystemAdapter:

const api = new ParseServer({
  databaseURI: databaseUri || 'mongodb://localhost:27017/dev',
  cloud: process.env.PARSE_SERVER_CLOUD || __dirname + '/cloud/main.js',
  appId: process.env.PARSE_SERVER_APPLICATION_ID || 'myAppId',
  masterKey: process.env.PARSE_SERVER_MASTER_KEY || '', 
  filesAdapter: new FSFilesAdapter({fileKey: process.env.PARSE_SERVER_FILE_KEY}), //Add your file key here. Keep it secret. Note that this is needed for Postgres or anyone using FSFilesAdapter. 
  ...
});

Be sure not to lose your key or change it after encrypting files. If you want to change your fileKey after encrypting, you will need the old fileKey and create CloudCode similar to what's mentioned below.

Note for those who already have unencrypted files using GridFSBucketAdapter or FSFilesAdapter adapters. You should look at #6768 for directions on how to encrypt your files.

This PR should not introduce any breaking changes

[codecov]

Codecov Report

Merging #6765 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6765      +/-   ##
==========================================
- Coverage   93.91%   93.88%   -0.04%     
==========================================
  Files         169      169              
  Lines       12029    12047      +18     
==========================================
+ Hits        11297    11310      +13     
- Misses        732      737       +5     

Impacted Files	Coverage Δ
src/Adapters/Files/GridFSBucketAdapter.js	77.64% <100.00%> (+6.00%)	⬆️
src/Controllers/index.js	96.66% <100.00%> (ø)
src/Adapters/Storage/Mongo/MongoStorageAdapter.js	92.85% <0.00%> (-0.67%)	⬇️
src/RestWrite.js	93.67% <0.00%> (-0.33%)	⬇️

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[dplewis]

LGTM!