Add command to get an attachment

[ced-b]

As gone over in #1078 here is a separate pull request on pulling attachment data.

The following command line command will write the Base64 string to the console:

signal-cli attachment --file-id=eBOlTUfsV4aD7KKXBnKr --file-name=cat.gif --content-type=image/gif

The following JSON RPC request will pull up the attachment in the following format:

{
    "jsonrpc": "2.0",
    "method": "attachment",
    "params": {
        "fileId": "eBOlTUfsV4aD7KKXBnKr",
        "fileName": "cat.gif",
        "contentType": "image/gif"
    }
}

Response:

{"jsonrpc":"2.0","result":{"dataBase64":"BASE64"}}

The actual command implementation was simple. However, the trick was to get access to the actual attachment through the attachment store. The best way I found was through the Manager instance passed to the command. To not have to expose all the guts of the manager etc. I wired this through the AttachmentHelper and to the AttachmentStore. Where this got a bit messy is the DbusManagerImpl as this would need to expose the function to get an attachment, but I did not see anything in that class for handling attachments really. Maybe you have a better idea of how to implement this.

[AsamK]

Looks good in general, I've added some comments for specifics.

Also please add the command to the man page: https://github.com/AsamK/signal-cli/blob/master/man/signal-cli.1.adoc

[ced-b]

@AsamK I'm just writing the doc but I'm not sure that calling the parameter recipient makes fully sense. Would it be originator?

I guess generally the external number is referred to as recipient so maybe it's fine, but just wanted to throw that out.

[AsamK]

@AsamK I'm just writing the doc but I'm not sure that calling the parameter recipient makes fully sense. Would it be originator?

I guess generally the external number is referred to as recipient so maybe it's fine, but just wanted to throw that out.

Yeah, recipient is not the best name here, but it's also used in the other commands. So I thiink it's ok to use it here as well to keep it consistent.

[AsamK]

Thanks a lot, looks good!