Add support for inline images and message priority to Send-MailKitMessage cmdlet

[MisterZedd]

Added a new InlineImages parameter of type Hashtable.

• Allows embedding images in the HTML body using content IDs.
• Uses MimePart with ContentDisposition = inline and adds them to BodyBuilder.LinkedResources.
• Create a hashtable like @{contentId = filepath} and add <img src="cid:contentId"> to the email body.

Added a new Priority parameter with allowed values: Normal, High, Low.

• Sets priority headers (X-Priority, X-MSMail-Priority, Importance) based on the selected priority.