Some inconsistencies in enum argument descriptions

**Describe the Issue:**

I have done some more investigation related to this PR: https://github.com/digitalocean/doctl/pull/1595, and I found more related issues / inconsistencies.

For instance in **doctl databases topics update**:

```
--cleanup-policy string Specifies the retention policy to use on log segments: Possible values are 'delete', 'compact_delete', 'compact' (default "delete")
--compression-type string Specifies the compression type for a kafka topic: Possible values are 'producer', 'gzip', 'snappy', 'Iz4', 'zstd', 'uncompressed' (default "producer")
```

but in **doctl databases create** and most other places:

```
--engine pg The database's engine. Possible values are: pg, `mysql`, `redis`, `mongodb`, `kafka` and `opensearch`. (default "pg")
```

In the first example, apostrophes don't interfere with command argument type generation and it stays defined as **string**.
In the second example, backticks are causing to use first value (**pg**) as data type for argument, which can be confusing.

I would be happy to create PR to fix this, however, I am not sure whether there's any reason attached to using backticks versus apostrophes. I am also not sure why default value is in quotes.

One possible solution is to introduce **enum** data type, and then argument descriptions could look like this:

```
--engine enum The database's engine. Possible enum values are: `pg`, `mysql`, `redis`, `mongodb`, `kafka` and `opensearch`. (default "pg")
```