Add accelerator_config to google_tpu_v2_vm

[modular-magician]

b/292585830

Follow-up to GoogleCloudPlatform/magic-modules#9308

This adds accelerator_config, which is a more flexible alternative to accelerator_type. Only one can be specified for creation, but both will be returned by the API.

The notable piece here is that we previously had a client-side default for accelerator_type, so adding the accelerator_config alternative without introducing a breaking change presented a challenge:

• Since the API does not accept both fields at once, the "v2-8" default needed to be removed, so that accelerator_config could be set.
• However, to continue supporting config without either of these fields specified the same way (default to "v2-8"), a customize diff was needed to conditionally populate the accelerator_type default.

Release Note Template for Downstream PRs (will be copied)

tpuv2: added`accelerator_config` field to `google_tpu_v2_vm` resource

Derived from GoogleCloudPlatform/magic-modules#9332