Buckets exported in templates don't include `shardGroupDuration` field

__Steps to reproduce:__

1. Create a bucket with a shard-group duration:
```
$ influx bucket create -n test -r 72h --shard-group-duration 1h
ID Name Retention Shard group duration Organization ID
099a4b957d8f7623 test 72h0m0s 1h0m0s dde64e34f10b74fb
```
2. Export the bucket in a template:
```
$ influx export --bucket-names test
apiversion: influxdata.com/v2alpha1
kind: Bucket
meta: null
spec:
name: test
retentionRules:
- everySeconds: 259200
type: expire
```

__Expected behavior:__
The shard-group duration to be included in the exported retention rule.

__Actual behavior:__
The shard-group duration isn't included.

Start with the code path behind `influx export --bucket-names`, focusing on how a bucket's retention rule is converted into the exported template. Reproduce the example and verify that the generated Bucket template includes the shard-group duration in its retention rule.

Written by the indexing model from the issue text.