Added auto generated Content ID header support

[rdelpeso]

• Added support for auto generated Content ID headers per attachment

Usage:

await client.send({
  // Other items...
  html:
  'First image attachment <img src="cid:attachment_id_0"/> Second image attachment <img src="cid:attachment_id_1"/>',
});

In support of #25

[rdelpeso]

Waiting for #26 before proceeding with this one.

[mathe42]

Looks good but I would add a option to the attachment object to set an ID as there might be use cases where you want to prepend your attachments allways with an extra attachment and then the IDs are not aligned.

So in the config add here

denomailer/config/mail/attachments.ts

Lines 3 to 6 in 7da737e

	interface baseAttachment {
	contentType: string;
	filename: string;
	}

id?: string | number

[mathe42]

I close this in favor of #28