* OpenAPI `generic_mount_paths` follow-up
An incremental improvement within larger context discussed in #18560.
* Following the revert in #18617, re-introduce the change from
`{mountPath}` to `{<path-of-mount>_mount_path}`; this is needed, as
otherwise paths from multiple plugins would clash - e.g. almost every
auth method would provide a conflicting definition for
`auth/{mountPath}/login`, and the last one written into the map would
win.
* Move the half of the functionality that was in `sdk/framework/` to
`vault/logical_system.go` with the rest; this is needed, as
`sdk/framework/` gets compiled in to externally built plugins, and
therefore there may be version skew between it and the Vault main
code. Implementing the `generic_mount_paths` feature entirely on one
side of this boundary frees us from problems caused by this.
* Update the special exception that recognizes `system` and `identity`
as singleton mounts to also include the other two singleton mounts,
`cubbyhole` and `auth/token`.
* Include a comment that documents to restricted circumstances in which
the `generic_mount_paths` option makes sense to use:
// Note that for this to actually be useful, you have to be using it with
// a Vault instance in which you have mounted one of each secrets engine
// and auth method of types you are interested in, at paths which identify
// their type, and for the KV secrets engine you will probably want to
// mount separate kv-v1 and kv-v2 mounts to include the documentation for
// each of those APIs.
* Fix tests
Also remove comment "// TODO update after kv repo update" which was
added 4 years ago in #5687 - the implied update has not happened.
* Add changelog
* Update 18663.txt
