Fix handling of context names with non-alphanumeric characters

Fix handling of context names with non-alphanumeric characters

Hex-encode most non-alphanumeric characters in kubectl context names
in Tramp file names.  This allows our Tramp method to work with
context names that contain characters which are not allowed in the
host name part of Tramp file names, such as ':'.

This (hopefully) fixes the issue reported at
https://lists.sr.ht/~eshel/kubed-devel/%3C87ldqz70a6.fsf@gmail.com%3E

* kubed-common.el (kubed-tramp-method): Bump to v2.
* kubed-tramp.el (kubed-tramp--unhex)
(kubed-tramp--decode-context-name)
(kubed-tramp--v2-context): New functions.
(kubed-tramp-context, kubed-tramp-enable): Adjust.
* kubed.el (kubed--hex-encoding-table)
(kubed--hex-allowed-chars-table): New constants.
(kubed--encode-context-name): New function.
(kubed-remote-file-name): Use it.