add services (closes #345)

[LukasGentele]

What issue type does this pull request address? (keep at least one, remove the others)
/kind feature

What is this pull request for? Which issues does it resolve? (use resolves #<issue_number> if possible)
resolves #345

Does this pull request has user-facing changes? (e.g. config changes, new/modified commands, new/modified flags)
yes, it:

• adds devSpace.services to config
• adds command devspace list service
• removes devSpace.sync[].resourceType (because this is always pod)
• devspace add package X will add X as service (maybe a flag could be implemented to control this behavior)

Does this pull request add new dependencies?
no

What else do we need to know?
additional todos could be to implement the convenience commands:

• devspace add service
• devspace remove service