Add mesh gateway WAN federation support

[cthain]

Changes proposed in this PR:

• Update the dev-server module to support WAN federation via mesh-gateways.
• Update the gateway-task module to allow datacenter federation across the WAN.
• Update the mesh gateway example:
  • Create two separate VPCs.
  • Create two separate Consul datacenters.
  • WAN federate the Consul datacenters via mesh gateways.
  • Demonstrate that a client service running in dc1 can successfully call a server service upstream running in dc2 via the mesh gateways.
• Note: This PR does not include support for ACLs. That will be added in a subsequent PR.

How I've tested this PR:

• Manually using the mesh-gateway example.

How I expect reviewers to test this PR:

:eyes:

Checklist:

• [ ] Tests added
• [ ] CHANGELOG entry added