kv: consider making the `kv.DB` API transactional

Currently, in production, SQL always uses the transactional KV API. The non-transactional `kv.DB` API is often used in tests and internal tools. There are a few reasons to consider making the `kv.DB` API transactional as well:

1. It currently fails to set admission headers: https://github.com/cockroachdb/cockroach/issues/79212.
2. Non-transactional requests are treated as idempotent in the `DistSender`, and it's possible that they get double-evaluated. This is somewhat documented in the comment below, but not widely understood by engineering teams that use the API.

https://github.com/cockroachdb/cockroach/blob/30c0ad2fc84cae90ed708f988f93e2c1ece271e3/pkg/kv/kvclient/kvcoord/dist_sender.go#L2860-L2864

We can either remove the `kv.DB` completely (which will require a lot of internal and test changes) or wrap each `db.Run` with a transaction.

Jira issue: CRDB-54910