Properly set the mount type for external storages

[rullzer]

@tobiasKaminsky there you go :)

This makes querying the mount-type on webdav actually useful.

to test:

1. Setup external storage
2. do a propfind with http://nextcloud.org/ns:mount-type

Beforw it is empty for external storages. Now it is nicely set to 'external'.

[codecov]

Codecov Report

Merging #8455 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #8455      +/-   ##
============================================
+ Coverage     51.75%   51.82%   +0.06%     
  Complexity    25363    25363              
============================================
  Files          1601     1601              
  Lines         95014    95014              
  Branches       1377     1377              
============================================
+ Hits          49178    49237      +59     
+ Misses        45836    45777      -59

Impacted Files	Coverage Δ	Complexity Δ
apps/files_external/lib/Lib/PersonalMount.php	18.18% <ø> (ø)	3 <0> (ø)	⬇️
apps/files_external/lib/Config/ConfigAdapter.php	0% <ø> (ø)	14 <0> (ø)	⬇️
lib/private/Security/CertificateManager.php	91% <0%> (-3%)	39% <0%> (ø)
apps/files_trashbin/lib/Trashbin.php	72.46% <0%> (-0.25%)	136% <0%> (ø)
lib/private/Server.php	83.28% <0%> (+0.09%)	282% <0%> (ø)	⬇️
lib/private/Files/Cache/Propagator.php	96.2% <0%> (+1.26%)	16% <0%> (ø)	⬇️
lib/private/Files/ObjectStore/SwiftFactory.php	54.43% <0%> (+54.43%)	30% <0%> (ø)	⬇️
lib/private/Files/ObjectStore/Swift.php	75% <0%> (+75%)	8% <0%> (ø)	⬇️

[tobiasKaminsky]

👍, working already with nextcloud/android#2206