Update scion dependency to get scionproto/scion#3640 which allows to get
rid of the clunky wrapper type for calling pathpol.Filter.

* Added support for used-defined path policies:

- ConnWrapper: wrapper around SCIONConn to allow for writing packets to paths that satisfy user-defined policy
- AppConf: contains path policy, path selection mode, and optional storage of static path and static overlay in case of staic path selection mode
- Running on SCIONLab, client fetches localhost address from gen/ia
- Running on local topology, client address is specificed using --client-address flag

Addressed PR comments:
- moved appconf and conn_wrapper to lib/scionutil
- replaced if-statements in WriteTo with switch statements
- removed code duplciation in DialSCION and DialSCIONWithConf in sssh.go and scionutils.go

* - rename connWrapper* to policyConn* and AppConf to PathAppConf
- policyConn now implements default behavior: arbitrary path selection
- for static and round-robin path selection, we have staticPolicyConn and roundRobinPolicyConn
- creating all policyConn* objects from PathAppConf
- moved AppConf and ConnWrapper back to the ssh/scionutils
- cleaned up commented code
- added log statements and doc strings

* embedded snet.Conn instead of snet.SCIONConn into policyConn, now get local IA via snet.DefNetwork

* - fixed import and DialSCION build issues
- added function pointer to policyConn to simulate "dynamic" method binding
- gofmt

* - changed pathSelectorFunc to pathSelector
- removed .String() from debug statements

* Added unit tests for policy_conn and path_app_conf

* fix build

* go fmt ./...

* More formatting.

Imports in blocks: first the std library, then other packages, then scionproto and netsec packages.

* Minor formatting

* Remove unnecessary function pointer in policyConn

Adapt UT to return working paths.

* added UT for staticPolicyConn

* Simplified design:

- One policyConn class
- Path selectors are subclassed
- Fixed intialization bug for pathResolver and localIA
- WriteTo defined on *policyConn
- NewPolicConn* return net.PacketConn
- Modified UTs to match new design

* go fmt

* Simplify code.

Construct a policy from the configuration only.

* go lint

* Minor fix, godoc for PathSelection type

* Add path filtering back, fix initialization, update tests

Re-implement path filtering by using pathpol.Filter.

Removed defaultPathSelector as the behaviour is virtually identical with
staticPathSelector now.

Separate path quering from path-selector initialization. This allows
for simpler testing of the path selection logic.
Initialize one path-selector instance per destination ISD-AS. This is
overkill for the current usage; it's clear however that a proper
PacketConn would need to do this, and implementing it is not much more
work than documenting a weird restriction. This would also allow to
eventually use this on the quic _server_.

Update / redo tests for static and round-robin path selectors.

* Update to scion master, remove pathpol.Path wrapper

Update scion dependency to get scionproto/scion#3640 which allows to get
rid of the clunky wrapper type for calling pathpol.Filter.

* Improve readability in filterPathSlice

* Improve method name for creating and initialising a selector

Co-authored-by: Juan A. García Pardo Giménez de los Galanes <juagargi@gmail.com>
Co-authored-by: Matthias Frei <matthias.frei@inf.ethz.ch>