ot::Srp::Server::SetAutoEnableMode() method

Enables/disables the auto-enable mode on SRP server. When this mode is enabled, the Border Routing Manager controls if/when to enable or disable the SRP server. SRP sever is auto-enabled if/when Border Routing is started it is done with the initial prefix and route configurations (when the OMR and on-link prefixes are determined, advertised in emitted Router Advert message on infrastructure side and published in the Thread Network Data). The SRP server is auto-disabled when BR is stopped (e.g., if the infrastructure network interface is brought down or if BR gets detached). This mode can be disabled by a `SetAutoEnableMode(false)` call or if the SRP server is explicitly enabled or disabled by a call to `SetEnabled()` method. Disabling auto-enable mode using `SetAutoEnableMode(false` call will not change the current state of SRP sever (e.g., if it is enabled it stays enabled).