Support container updates

First off: this issue is different than creating LDPC on route to creating an LDP-RS as a member of an LDPC that doesn't exist yet. That particular case was covered in https://github.com/solid/node-solid-server/pull/513.

This issue is about creating or updating an LDPC (ie an LDP-BC, at the time of this writing). It will conform to LDP provided that the information in the update payload doesn't get in the way of changing the containment triples ( https://www.w3.org/TR/ldp/#h-ldpc-put-mbrprops ). So, for example, adding a label to a container (<> rdfs:label "Foo") would be fine, but PUT/PATCH that inserts or deletes ldp:contains relations should not succeed.

The use cases are generally about including additional "metadata" as part of the container description. It is partly discussed in https://github.com/solid/node-solid-server/issues/547 .

If this feature is implemented, https://github.com/solid/node-solid-server/issues/547 can be revisited, and as far as I can tell, the feature where updating a container's description through /.meta can be removed altogether (as interacting directly with the URI in question would be generally preferable.)

Edit: rel="describedBy" .meta in HTTP header can also be dropped as the interaction will only go through container URI, and that .meta will only be internal to the server.

首先查看 LDP 容器更新要求以及 issue #547 中的相关讨论，然后定位处理 LDPC URI 更新的服务器入口点。确定 PUT 和 PATCH payload 如何处理 ldp:contains 三元组，以及如何处理其他元数据。完成的标准是：可以通过容器 URI 更新容器元数据，同时不允许更改包含关系，并且不再需要所描述的 .meta 行为。