[Feature Request] Add support for attachment caption

[Thireus]

Caption is currently implemented for ReceiveMessageHandler.java:

signal-cli/src/main/java/org/asamk/signal/ReceiveMessageHandler.java

Line 592 in fab1b96

writer.println("Caption: {}", attachment.caption().get());

And also implemented here:

signal-cli/src/main/java/org/asamk/signal/json/JsonAttachment.java

Line 22 in fab1b96

final var caption = attachment.caption().orElse(null);

While the attachment JSON format supports it, it is currently not possible to set caption for uploaded attachments as it is missing from JSON RPC, CLI options and DBUS. And the function that would need to parse the caption (if any is attached) is defined here:

https://github.com/AsamK/signal-cli/blob/master/lib/src/main/java/org/asamk/signal/manager/util/AttachmentUtils.java#L30-L40

I propose a new --attachment-metadata (-am) vector option where the user could define the additional metadata for each attachment. For example, if 4 attachments are present:

-am '[{"caption":"image1_caption"}, None, {"width":800, "height":600}, {"caption":"image4_caption", "width":800, "height":600}]'

• attachment 1 would receive caption "image1_caption"
• attachment 2 has no additional metadata
• attachment 3 has width set to 800 and height set to 600
• attachment 4 has caption "image4_caption" and has width set to 800 and height set to 600

This would allow not only to support caption but also additional metadata such as voiceNote, borderless, preview, width, height, custom_filename, etc.