Add browserless support

[SnowyLeopard]

Topic and Scope

Inspired by https://github.com/nextcloud/cookbook/pull/2622 this adds support for the use of browserless(.io). This allows fetching recipes from websites that need javascript to be executed and bypasses bot detection for some websites.

Concerns/issues

• I haven't figured out yet how to store the token securely, now it is just saved as plain text in the DB.
• Does this need more readme/how to documentation?

Formal requirements

There are some formal requirements that should be satisfied. Please mark those by checking the corresponding box.

• [x] I did check that the app can still be opened and does not throw any browser logs
• [x] I created tests for newly added PHP code (check this if no PHP changes were made)
• [x] I updated the OpenAPI specs and added an entry to the API changelog (check if API was not modified)
• [ ] I notified the matrix channel if I introduced an API change

[provokateurin]

I haven't figured out yet how to store the token securely, now it is just saved as plain text in the DB.

You can make appconfig values sensitive, then they are automatically stored in an encrypted form in the DB.