Replace strings with const values

[Robbie-Microsoft]

Core Library

MSAL Node (@azure/msal-node)

Wrapper Library

Not Applicable

Public or Confidential Client?

Confidential

Description

An EncodingTypes enum has been introduced in this PR.

Every instance of "hex" should be replaced with this new constant, and a "base64" value should be added to the EncodingTypes enum as well.