Allow requesting a specific depth of spaces, starting from a specific space

The following endpoint:

/wikis/{wikiName}/spaces/{spaceName}

returns all the spaces of a wiki, including the nested one.

The following endpoint:

/wikis/{wikiName}/spaces/{spaceName}/spaces/

is undocumented but has the same behavior.

It would be useful to be able to request only one level of spaces, or a specify a depth, and the space of which the children should be listed.

Suggested parameters:

 - nested=true|false
 - depth=N (1 for one level)

To specify the space of which the children should be listed, either:

• We change the behavior of /wikis/{wikiName}/spaces/{spaceName}/spaces/, which currently does not make much sense but that would break retro-compatibility (are there people really using it though?)
• We add a new parameter (from? space?)
• We add a new endpoint
• something else?

Affected versions actually probably every versions since nested spaces were introduced.

Context: https://matrix.to/#/!ikPtGZaGWtyblizzlR:matrix.xwiki.com/$16747484021237ftQTw:matrix.xwiki.com?via=matrix.org&via=matrix.xwiki.com&via=progressiv.dev