refactor(files_external): migrate Ajax with OC_JSON to proper controller

[susnux]

Summary

Another place to get rid of legacy stuff 🧹

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[come-nc]

failures related

There were 4 failures:

1) Test\UrlGeneratorTest::testLinkToDocRoot with data set #0 ('files_external', 'ajax/oauth2.php', [], '/index.php/apps/files_externa...h2.php')
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'/index.php/apps/files_external/ajax/oauth2.php'
+'/files_external/ajax/oauth2.php'

/home/runner/work/server/server/tests/lib/UrlGeneratorTest.php:79

2) Test\UrlGeneratorTest::testLinkToDocRoot with data set #1 ('files_external', 'ajax/oauth2.php', ['trat', 'dat'], '/index.php/apps/files_externa...ut=dat')
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'/index.php/apps/files_external/ajax/oauth2.php?trut=trat&dut=dat'
+'/files_external/ajax/oauth2.php?trut=trat&dut=dat'

/home/runner/work/server/server/tests/lib/UrlGeneratorTest.php:79

3) Test\UrlGeneratorTest::testLinkToSubDir with data set #0 ('files_external', 'ajax/oauth2.php', [], '/nextcloud/index.php/apps/fil...h2.php')
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'/nextcloud/index.php/apps/files_external/ajax/oauth2.php'
+'/nextcloud/files_external/ajax/oauth2.php'

/home/runner/work/server/server/tests/lib/UrlGeneratorTest.php:90

4) Test\UrlGeneratorTest::testLinkToSubDir with data set #1 ('files_external', 'ajax/oauth2.php', ['trat', 'dat'], '/nextcloud/index.php/apps/fil...ut=dat')
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'/nextcloud/index.php/apps/files_external/ajax/oauth2.php?trut=trat&dut=dat'
+'/nextcloud/files_external/ajax/oauth2.php?trut=trat&dut=dat'

/home/runner/work/server/server/tests/lib/UrlGeneratorTest.php:90

[come-nc]

update-baseline is missing. And /compile?