Support image input to ellama-instant

[swapneils]

This PR uses the media API in llm to support sending multipart inputs (which can contain both text and images) as lists into ellama-instant. It also adds ellama-summarize-image to call ellama-summarization-provider on an image file.

NOTE: The media API is apparently still alpha quality. I'll leave to your judgement whether this PR should be merged now or wait on llm's media support stabilizing.

[swapneils]

Relatedly, looks like the threshold for FSF copyright assignment is 15 lines? #335 (comment)

Per their website (https://www.fsf.org/licensing/assigning.html) the maintainer needs to make a questionnaire to determine eligibility. Could you share that to unblock this PR?

[s-kostyaev]

https://www.gnu.org/software/emacs/manual/html_node/emacs/Copyright-Assignment.html

etc/copyright-assign.txt:

Please email the following information to assign@gnu.org, and we
will send you the assignment form for your past and future changes.

Please use your full legal name (in ASCII characters) as the subject
line of the message.
----------------------------------------------------------------------
REQUEST: SEND FORM FOR PAST AND FUTURE CHANGES

[What is the name of the program or package you're contributing to?]
Emacs

[Did you copy any files or text written by someone else in these changes?
Even if that material is free software, we need to know about it.]


[Do you have an employer who might have a basis to claim to own
your changes?  Do you attend a school which might make such a claim?]


[For the copyright registration, what country are you a citizen of?]


[What year were you born?]


[Please write your email address here.]


[Please write your postal address here.]





[Which files have you changed so far, and which new files have you written
so far?]

[s-kostyaev]

TODO: add this information to readme.