Feature: remove requirement to specify cluster's internal IAM role name

**Is your feature request related to a problem? Please describe.**
When using workload identity to backup a database to s3, you have to specify two roles: the Cockroach-managed role and the customer-controlled role to assume:

From docs here: https://www.cockroachlabs.com/docs/v24.1/cloud-storage-authentication#run-the-operation-from-a-cockroachdb-dedicated-cluster:

```sql
BACKUP DATABASE {database} INTO 's3://{bucket name}/{path}?AUTH=implicit&ASSUME_ROLE=arn:aws:iam::{AWS account ID}:role/crl-dr-store-user-{cluster ID suffix},arn:aws:iam::{account ID}:role/{operation role name}' AS OF SYSTEM TIME '-10s';
```

**Describe the solution you'd like**
I'd like to omit the cockroach-controlled role and just specify:

```sql
BACKUP DATABASE {database} INTO 's3://{bucket name}/{path}?AUTH=implicit&ASSUME_ROLE=arn:aws:iam::{account ID}:role/{operation role name}' AS OF SYSTEM TIME '-10s';
```

so that I don't have to query the cockroach cloud API and manually construct a role name.

Jira issue: CRDB-40057