Add token helper functions to oauth2-client module

[MarcialRosales]

Proposed Changes

Add function that parses a token and extract the expiration time.

These functions are required by those client applications which need to refresh the access token and do that they need to know the expiration time.

Types of Changes

What types of changes does your code introduce to this project? Put an x in the boxes that apply

• [ ] Bug fix (non-breaking change which fixes issue #NNNN)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• [ ] Documentation improvements (corrections, new content, etc)
• [ ] Cosmetic change (whitespace, formatting, etc)
• [ ] Build system and/or CI