"Secret token" setting should have string type

[axw]

APM Server version (apm-server version):

7.11.0 BC1, integration 0.1.0-dev2

Description of the problem including expected versus actual behavior:

In the APM integration settings, the "secret token" setting is a toggle (bool). It should be a free-form text entry.

See also #4542 (comment)

Steps to reproduce:

1. Install the "apm" integration in Fleet
2. Navigate to the integration settings, expand "Colllect application traces"
3. Secret token is a toggle, but should be a text entry like for "Host":

[jalvz]

Created an apm integration with version 0.1.0-dev.3, this shows a text box now.