* Begin moving XContent to a separate lib/artifact
This commit moves a large portion of the XContent code from the `server` project
to the `libs/xcontent` project. For the pieces that have been moved, some
helpers have been duplicated to allow them to be decoupled from ES helper
classes. In addition, `Booleans` and `CheckedFunction` have been moved to the
`elasticsearch-core` project.
This decoupling is a move so that we can eventually make things like the
high-level REST client not rely on the entire ES jar, only the parts it needs.
There are some pieces that are still not decoupled, in particular some of the
XContent tests still remain in the server project, this is because they test a
large portion of the pluggable xcontent pieces through
`XContentElasticsearchException`. They may be decoupled in future work.
Additionally, there may be more piecese that we want to move to the xcontent lib
in the future that are not part of this PR, this is a starting point.
Relates to #28504
Alpar Torok
Lee Hinman