Custom endpoints

[big-andy-coates]

Currently Kafka endpoints are read from environment variables and the JSON schema serde under development likewise reads the Schema Registry endpoint from environment variables.

Both of these should be customisable. Currently this is overridden via the context builder. This is wrong, as its incompatible with system tests, which require the endpoints to be read from environment properties.

We should:

1. Provide a mechanism for injecting new (secure) endpoint providers for both Kafka and SR, e.g. an environment var that points to a custom implementation to use. (The system tests won't set this new env var, so services will default to loading end points from env vars, which is compatible with system tests).
2. Remove the ability of customising this on the service context builder, as its misleading. This will still be needed internally, as we'll need in inject mocks for unit testing. (e.g. TestKafkaStreamsExtensionOptions). But it should be moved to an internal interface / type.

[big-andy-coates]

Second part is done.