Support multipart/form-data in platform-http extension

[ppalaga]

I assume we definitely want to support this, for the platform-http to be on par with the older HTTP components.

The question is how to implement that. The older components use AttachmentMessage from camel-attachments to store the uploaded files. camel-attachments depends on javax.activation.

(a) The "easy" solution: pull camel-attachments and javax.activation as dependencies, and do not care for the increase of the app size caused by that.

(b) The "optional" solution: provide some no-op handling of file uploads by default that does not depend on camel-attachments and javax.activation. Handle the uploads properly via camel-attachments (wrapped in an extension) and javax.activation only if the user adds them as deps.

(c) ?

Is the work associated with (b) worth the benefits it might bring?

[ppalaga]

I implemented the "easy" solution and it adds 560 kB to the master baseline in the platform-http itest.