Refactor nft front-end to properly parse bytecode

[qdeslandes]

Plenty of new features have been introduced during H2 2024 which are supported by bfcli. The nft front-end hasn't progressed at the same speed due to many limitations with its current implementation, making bpfilter unusable with nftables.

The front-end's logic is the following:

• nftables CLI binary parse the user-define rules and converts them into nft bytecode
• libbpfilter (linked to nftables) send the bytecode to the bpfilter daemon using Netlink messages
• The nft front-end in the daemon parses the Netlink messages to extract the nft bytecode, and converts the bytecode into its internal format (shared by all the front-ends)

The current implementation of the bytecode parsing logic can't be scalable to understand more complex filtering rules and only works with simple "filter IP xxx.xxx.xxx.xxx" rules. It must be refactored to provide a generic framework to properly parse the bytecode and allow for more matchers and rules to be supported in the future.

Some exploration is required to understand nft bytecode format, investigate if a similar tool or library already exists, and suggest possible solutions for this issue.

The purpose of this task is to define, document, and implement a scalable framework to parse nft bytecode, not to reach feature parity between nftables and bfcli.