WIP Stretch cluster support

[vuldin]

This PR adds the ability to configure helm deployments so that Redpanda can form a stretch cluster. A few notes:

• allows enabling RPC communication by external hostnames
• allows adding additional seed servers so subsequent deployments can reach out to already existing deployments to find leader
• new parameters are intentionally left out of the values.yaml (but they should likely be added to the schema regardless). The reason is because modifying additionalSeedServers or enabling external RPC is error prone and is difficult to configure. The goal is to enable setting these values but then provide a guide showing how a tool like Crossplane could be used to manage multiple helm deployments (and managing the proper values for these new parameters in values.yaml).

TODOs:

• external hostname must be resolvable, so I've ensure this is the case by updating the hosts file on Redpanda pods with the external hostnames and LB IP addresses
• this is the start of other work, including an investigation and PoC with Crossplane