[Prototype] nftables flowtable support

[caseydavenport]

Description

This is a proof-of-concept PR for implementing nftables flowtable support in Calico. flowtables allow bypassing of large parts of the Linux networking stack for established connections, which should theoretically substantially improve the performance especially for longer lived connections.

Some key things to consider here:

• How does this interact with a potential equivalent implemention in kube-proxy?
• How does this perform in clusters with larger numbers of short-lived connections?
  • Does flowtable overhead actually hurt those environments?
  • Do we need to be able to include / exclude certain endpoints or flows from this optimization?
  • Should it be configurable via e.g., policy?

Related issues/PRs

Kube-proxy implementation: kubernetes/kubernetes#128392

Todos

• Tests
• Documentation
• Release note

Release Note

Support for flowtables in nftables data plane for improved connection performance. 

Reminder for the reviewer

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