server: change prototype of add_service()

server: change prototype of add_service()

To easily add new methods to a service, pass all the methods
through a struct.
While there, drop the typedef for the methods and add currently
unused new methods to support keep-alive and connections during
keep-alive.

No change in functionality.

Change-Id: I2b5e7140db95021f6e7201e9d631ee340c60b453
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6838
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins