add parachain c

[zqhxuyuan]

some parachain may not use GeneralKey as its token, but use just its parachain, i.e. PHA:

https://github.com/AcalaNetwork/Acala/blob/d92b2db7e384ebf24dafc89d7cc0e04ee4724435/runtime/karura/src/lib.rs#L1811-L1812

// Phala Native token
Token(PHA) => Some(MultiLocation::new(1, X1(Parachain(parachains::phala::ID)))),

this PR add parachain c, and use parachain(3) to represent its token.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.74%. Comparing base (4a66b29) to head (f87e99f).
⚠️ Report is 222 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #699      +/-   ##
==========================================
+ Coverage   74.63%   74.74%   +0.11%     
==========================================
  Files          83       83              
  Lines        7261     7293      +32     
==========================================
+ Hits         5419     5451      +32     
  Misses       1842     1842              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[shaunxw]

Parachains in mock shares the same CurrencyIdConvert, this will make all of them convert Here to CurrencyId::C, which doesn't make sense.

You'll need to define a convert impl only for Parachain C.