[Feature Request] Add support for a configurable salt prefix

[parametalol]

Is your feature request related to a problem? Please describe.

• Shadowsocks communication with default parameters is blocked by some providers.

Describe the solution you'd like

• The issue can often be solved by adding a custom salt prefix, as the new Outline client does.

Describe alternatives you've considered

• Changing port doesn't help. Adding POST%20 prefix helps.

Additional context

Preferably, the prefix value should be taken from the prefix URL query parameter, for example:

ss://[email protected]:12345/?prefix=POST%20

Please consider shadowsocks/shadowsocks-org#204. I'd envisage displaying a warning when activating the feature.