feat(access-token): add ephemeral access-token resource

[h3adex]

Description

Internal Issue: STACKITTPR-353

Checklist

• [x] Issue was linked above
• [x] Code format was applied: make fmt
• [x] Examples were added / adjusted (see examples/ directory)
• [x] Docs are up-to-date: make generate-docs (will be checked by CI)
• [x] Unit tests got implemented or updated
• [x] Acceptance tests got implemented or updated (see e.g. here)
• [x] Unit tests are passing: make test (will be checked by CI)
• [x] No linter issues: make lint (will be checked by CI)

[h3adex]

One general question I still have: What happens when the user authenticates via token flow?

provider "stackit" {
  default_region        = "eu01"
  service_account_token = var.service_account_token
}

Will look into that tomorrow.

[h3adex]

Error message when you try to generate a access token without a valid config

[rubenhoenle]

Error message when you try to generate a access token without a valid config

For me it must be at least stated in the docs which requirements exist to use this resource: User must use keyflow for authentication. Or am I wrong here?

[h3adex]

E2E Tests