* add Istio Gateway Source
* add documentation for Istio Gateway Source
* make both istio namespace and ingress gateway service configurable
* prefix gateway types, constructors, and flags with 'istio-'
* fix: add missing sources to source flag docs
* ref(sources): refactor source registration and lookup to be lazy.
* fix(fake): don't make changes to passed in config values
* rework without init, tests are missing
* make client provider public
* fix all tests
* change parameter list order, minor improvements
* clientprovider -> clientgenerator, switch naming for interface/struct
* ref(source): use testify with mocks in test code
* fix: re-introduce NewMockSource for convenience
* fix: avoid circular dependency
* ref: increase usage of testify
* chore: vendor testify as a dependency
* fix(*): cleanup testify expectations
