mongodb should be a dependency or peerDependency instead of devDependency

[crccheck]

In /src/cache, dataloader and bson are dependencies, but mongodb is not:

apollo-datasource-mongodb/src/cache.js

Lines 1 to 3 in 08e1c07

	import DataLoader from 'dataloader'
	import { ObjectId } from 'mongodb'
	import { EJSON } from 'bson'

Current dependencies:

apollo-datasource-mongodb/package.json

Lines 19 to 26 in 5d8f93a

	"dependencies": {
	"@types/mongodb": "^3.5.27",
	"apollo-datasource": "^0.3.1",
	"apollo-server-caching": "^0.3.1",
	"apollo-server-errors": "^2.4.1",
	"bson": "^4.1.0",
	"dataloader": "^1.4.0"
	},

That means if you install apollo-datasource-mongodb, then run npm prune --production, there's a chance that the mongodb package will be pruned and this package will error with:

Error: Cannot find module 'mongodb'

[crccheck]

what's strange is I have mongoose as a dependency, so I should be getting mongodb via that: https://github.com/Automattic/mongoose/blob/b0043b7be02f846bb224e2332be0867aec058904/package.json#L24

edit:
I had mongoose which wanted mongodb@4.5.0 and mongodb-memory-server which wanted mongodb@^4.5.0, which resolved to v4.7.0. So when my build process ran npm prune, it deleted node_modules/mongodb as it should have and my error happened

[lorensr]

peerDependency sounds good, PR welcome