sstp-client: does not obey the interface metric

Maintainer: @vgaetera @fededim
Environment: Any, for example, TP-Link Archer C7 v2 on OpenWrt 23.05.5

Description:

The sstp.sh protocol handler implements all option parsing from scratch instead of inheriting all common PPP option logic. This, for example, leads to the configured interface metric being ignored.

Example of an affected configuration:

config interface 'vpn'
	option proto 'sstp'
	option server 'sstp.vpn.example'
	option username 'username'
	option password 'password'
	option ipv6 '1'
	option log_level '0'
	option peerdns '0'
	list dns '8.8.8.8'
	list dns '8.8.4.4'
	option metric '91'

This results in the created route having no explicit metric, as opposed to metric 91.

A workaround exists, but this should not be necessary:

	option pppd_options 'defaultroute-metric 91'

Please look at how pppossh includes standard functions from ppp.sh and implements its protocol handler on top of that.

Start by comparing the sstp.sh protocol handler with pppossh and the shared ppp.sh functions, as described in the issue. Verify the affected configuration produces a route with metric 91 rather than no explicit metric, and confirm that standard PPP option handling is used without requiring pppd_options.