Set config "extensions" option when converting ref storage format

Set config "extensions" option when converting ref storage format

When converting to reftable format the option extensions.refStorage must
be set to "reftable" [1]. When converting back to refdir format this
config option needs to be removed.

Introduce constants for refStorage config options, also for the
"reftree" format.

[1] https://git.eclipse.org/r/plugins/gitiles/jgit/jgit/+/master/Documentation/technical/reftable.md#Version-1

Change-Id: I190222fa5edc1ad7309daa9be17ca934ff7971e3
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>