Add support for extensible program context

Today bpf2c computes hash on full `ebpf_ctx_descriptor_t`, that prohibits any change to the context descriptor, even if the changes are non-breaking, and new fields are being added at the end (same as done in Linux)

**Proposal**
1. Only include `data`, `end`, and `meta` in the hash, and also store the context size in native module.
2. ebpf runtime will compute and compare the hash using only the above 3 fields.
3. ebpf runtime will also ensure the context size in native module <= context size at runtime
4. It is extension's responsibility to ensure it does not break the offsets of existing fields when extending the context (same as Linux)