check if iso exists before downloading

#### Description

Check to see if an ISO already exists before performing download.

#### Use Case(s)
Unusual 27m download that usually takes only a couple ... suddenly makes this sound like a much better feature.

```
2024/10/15 22:56:48 packer-plugin-proxmox_v1.2.1_x5.0_linux_amd64 plugin: 2024/10/15 22:56:48 Acquiring lock for: https://releases.ubuntu.com/22.04.5/ubuntu-22.04.5-live-server-amd64.iso?checksum=sha256%3A9bc6028870aef3f74f4e16b900008179e78b130e6b0b9a140635434a46aa98b0 (downloaded_iso_path/c968bbbeb22702b3f10a07276c8ca06720e80c4c.iso.lock)
proxmox-iso.ubuntu-2204: ubuntu-22.04.5-live-server-amd64.iso 677.64 MiB / 1.99 GiB [===========================>--------------------------------------------------------] 33.25% 27m14s

```

#### Potential configuration

```
Similar to pulling an image with kubernetes:
- isoPullPolicy: Always
- isoPullPolicy: IfNotPresent
```

#### Provider specific?

Makes sense this might be provider specific within the image-builder project I'm using. If this already exists as part of the SDK and I just need to implement it in the provider I'm using I can give it a shot, if you could just point me in the right direction ... (hopefully this is the right place for this ticket).