identity: configure access tokens TTL

## Jobs to be done (optional)

When a user creates a new client via the API, the default TTL for access tokens for that client inherited the default for new clients of 3600 seconds, with no way of overriding it. The duration of the token can now be customized from the UI, the API and Terraform.

## Definition of done

Update this page to show how to configure the TTL from the UI, the API and Terraform.

## Information

- [Aha](https://konghq.aha.io/features/KID-20)
- API : [confirm version](https://kongstrong.slack.com/archives/C0A2G9U5CTX/p1786963504562409)
- [dev API PR](https://github.com/Kong/platform-api/pull/2346)

## Due date (optional)

## Size

Rough estimate of effort:
M