Inline images in SendGridSender

[chakrygin]

Hi!

I`m trying to send email containing images using SendGridSender. But the images are not displayed in gmail and some other web cleints.

The raw SendGridClient supports the Disposition property (https://github.com/sendgrid/sendgrid-csharp/blob/main/src/SendGrid/Helpers/Mail/Model/Attachment.cs#L34-L38), which works fine. Unfortunately this property is not used in SendGridSender. And the IsInline property is not used at all.

Maybe this method should use the property like this?
https://github.com/lukencode/FluentEmail/blob/master/src/Senders/FluentEmail.SendGrid/SendGridSender.cs#L141-L146

 private async Task<SendGridAttachment> ConvertAttachment(Core.Models.Attachment attachment) => new SendGridAttachment
 {
 Content = await GetAttachmentBase64String(attachment.Data),
 Filename = attachment.Filename,
 Type = attachment.ContentType,
 Disposition = attachment.IsInline ? "inline" : "attachment",
 };

[crowz4k]

@lukencode I am willing to fix this issue with PR, but I wonder if you will approve it?
One more thing missing here is ContentId