Add `insecure` to the otlp interface provider databag

[MichaelThamm]

Why is this important?

The current implementation of the OTLP interface lib is that the client side (requirer) determines if the communication is secure via the provider's endpoint scheme. This has worked so far because we have only implemented this for the opentelemetry-collector charms which use the otlp exporters allowing this behavior. However, the gRPC spec states that you only need:

ipv4:address[:port][,address[:port],...] -- IPv4 addresses

Steps to resolve

1. The provider allows the charm author to define the entire endpoint; no changes to the add_endpoint API is needed.
2. We use the root store for communication in most charms, so there is no need to communicate CA certs in the databag.
3. We do need to update the OtlpEndpoint model to include the insecure information since secure communication is scoped per endpoint.
   • We do not need to update the requirer because the endpoints API offers the list of OtlpEndpoint which now has extra insecure context for the charm author to do with as they please.