Backports #34612

Prior this patch, using the `--format=kubernetes` option with
`tctl auth sign --tar` would crash due to the filesystem abstraction
used to capture the `tctl` output files did not support removing or
`stat`ing files.

In addition, the kubeconfig file writer did not use the filesystem
abstraction given to the identity file writer, but would only write
files out to the host filesystem. This means that any kubeconfig
file oututs would not be included in the output tarfile stream.

This patch:
 * Updates the tarfile filesystem abstraction to buffer files created
   by `tctl` until the write is complete, and then stream the archive
   out at the end. This gives sensible semabtics to the remove and
   stat operations.
 * Updates the kubeconfig writer to take a filesystem abstraction
   compatible with the one used by the identity file writer, so that
   the kubeconfg file output is correctly caught by the tarfile writer.

Fixes: #34371
Changelog: Fixes crash when writing kubeconfig with `tctl auth sign --tar`

* Fix backport test detritus