[STH] Instance stop and kill body params fix

[ErykSol]

Currently instance endpoints /_kill and /_stop requires to pass in body InstatnceStatus Code to succesfully pass request validation which makes it impossible to use by regular user.

This PR changes required body params to be optional, and changes reqest body format from:
[InstanceStatus.SomeCode, {...requiredParams}]
to
{...optionalParams}

Review checks:

These aspects need to be checked by the reviewer:

• Verify and confirm operation
• All STH tests pass
• Documentation is updated or no changes

Verification:

1. /_stop
   Invalid timeout format:
   

Invalid canCallKeepalive format:

Valid params provided:

No params provided (use of default params):

2. /_kill
   Invalid params format:
   

Valid params provided:

No params provided (use of default params):