Making return types for functions excplicit

[kbsaid]

Adding a new eslint rule for our typescript backend projects:

"@typescript-eslint/explicit-function-return-type": "error"
A lot of our backend solutions don't have any explicit rule for return types for our functions. This makes at times harder to read what functions actually return and can potentially cause confusion. There is a reason to why we use typeScript in the backend, and mixing it with non types makes often times our backend solution not consistent with our Guide of how we develope and write code in 24sevenoffice