Support for External DNS names

[EronWright]

Description

The goal is to assign an external DNS name to each segment store, compatible with the External-DNS component. Given a domain suffix for the cluster, each segment store should be assigned a reasonable name, e.g. <name>.<domain> where <name> is the segment store pod name, and <domain> is a value provided by the PravegaCluster spec. The assigned name should become the 'advertised' address of the segment store for use by the Pravega client.

The motivation is to support TLS connections to segment store with hostname verification enabled. Related to pravega/pravega#3899.

Suggestions for an improvement

Approach:

1. add a new field to the PravegaCluster spec, e.g. ingress/domain.
2. When creating a Service for a given SegmentStore, compute an FQDN as follows: <name>.<domain>. (note the period after the suffix).
3. Record the FQDN as an annotation on the Service resource. The annotation should have the key external-dns.alpha.kubernetes.io/hostname, to be compatible with the External-DNS component.
4. Adjust the Pravega entrypoint script to pick up the annotation when present (see Use external DNS name as segment store advertised endpoint pravega#3899).

The expected result is:

1. the External-DNS component creates a DNS record for the segment store.
2. the segment store advertises the correct DNS name to clients.
3. client is able to make a TLS connection with hostname verification enabled.