externalstore: unbreak writes to non-default storage clusters due to isReadOnly()

externalstore: unbreak writes to non-default storage clusters due to isReadOnly()

Instead of telling ExternalStoreMedium classes the default write stores and using
that to make them read-only, let them be configured via other means. For example,
ExternalStoreMwstore already respects FileBackend::isReadOnly() for each location
(e.g. file backends) and ExternalStoreDB checks LoadBalancer::getReadOnlyMode()
for each location (e.g. DB cluster).

Make ExternalStoreAccess::isReadOnly() take a list of base URLs, default to the
default write stores if not specified.

Bug: T227156
Change-Id: I3161890fb2ccb46d6206628f0cd88f8af9f1688c
Follows-Up: I40c3b5534fc8a31116c4c5eb64ee6e4903a6197a