Add versioning to vets-api module

[kjduensing]

Description

The Travel Pay API has versioning in place to reduce downstream churn when endpoints change. In order to take advantage of this we should implement versioning on vets-api (and correspondingly vets-website). This will result in calling vets-api endpoints that look something like this:

• GET staging-api.va.gov/travel_pay/v1/claims
• POST api.va.gov/travel_pay/v2/claims/mileage

Acceptance Criteria

• [ ] Current endpoints restructured within version 1, e.g. v1
• [ ] Architecture docs exists
  • [ ] showing how this versioning scheme can scale
  • [ ] showing how this versioning scheme can map to any travel pay api versioning numbers (e.g. they are currently on v1.1, but I'd recommend sticking with v1, v2, v3, etc)

Definition of Done

• [ ] All acceptance criteria are met
• [ ] Documentation is updated

[ayush-chak]

not necessary for rollout, but we will do this after rollout

[hinzed1127]

@kjduensing is there something more complex desired than the initial versioning in place, or is this ticket outdated now?