XML-RPC - add updateMedia endpoint

[joedolson]

In #58582, @thomashorta reported on alt attributes missing from the XML RPC endpoints. Fetching that data was fixed in [56637], but adding an endpoint to update media that specifically handles media data like alt attributes requires a more substantial enhancement.

From #58582:

Updating MediaItems through the XML-RPC API is also done directly by the Posts API, more specifically through wp.editPost (codex), which also doesn't support an alt field. This makes sense, as this API is more generic, but at the same time, there are no specific updateMedia methods in the Media API.

Suggestion for updating
I'm not sure about this one, but I think a new method in the Media API would be needed (e.g.: wp.updateMediaItem) to properly manipulate a specific input struct and call the Post update functions internally.

[joedolson]

Given the mostly legacy status of XML RPC in WordPress, this doesn't seem like a high priority. However, if anybody find themselves needing this, I'd be happy to address it. Based on my working guesstimate of usage of XML RPC, however, it doesn't seem like something to focus on at this time.

[joedolson]

Given the mostly legacy status of XML RPC in WordPress, this doesn't seem like a high priority. However, if anybody find themselves needing this, I'd be happy to address it. Based on my working guesstimate of usage of XML RPC, however, it doesn't seem like something to focus on at this time.

[alh0319]

@joedolson After lots of frustration with the WP Android app and digging through issues to try and figure out why the alt text I entered doesn't save, I ended up here.

This ticket from the Android App (​https://github.com/wordpress-mobile/WordPress-Android/issues/8393) suggests that it can't be fixed until there's support for alt in the XML-RPC API. That might make this a higher priority, given the app's usage.

Trac ticket: https://core.trac.wordpress.org/ticket/59414

[joedolson]

Thanks for the PR, @sukhendu2002!