aya-ebpf: add ifindex support to XdpContext

[gth828r]

This change exposes the ifindex field from the underlying xdp_md data structure to the XdpContext in Aya. The ifindex represents the unique OS-provided index for a network interface.

Fixes #1140

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[gth828r]

For complete transparency, I should mention that when testing on my machine all of the unit tests passed, but a few of the integration tests failed. This was the case on both the main branch and my feature branch. Below are the set of failed tests:

failures:
    tests::btf_relocations::relocation_tests::_enum_signed_32_checked_variants_true_some_kernelversion_new_6_0_0_https_github_com_torvalds_linux_commit_6089fb3_0x7bbbbbbbi32_as_u64_expects
    tests::btf_relocations::relocation_tests::_enum_signed_32_true_some_kernelversion_new_6_0_0_https_github_com_torvalds_linux_commit_6089fb3_0x7bbbbbbbi32_as_u64_expects
    tests::btf_relocations::relocation_tests::_enum_signed_64_checked_variants_true_some_kernelversion_new_6_0_0_https_github_com_torvalds_linux_commit_6089fb3_0xcccccccdddddddi64_as_u64_expects
    tests::btf_relocations::relocation_tests::_enum_signed_64_true_some_kernelversion_new_6_0_0_https_github_com_torvalds_linux_commit_6089fb3_0xcccccccddddddddi64_as_u64_expects
    tests::btf_relocations::relocation_tests::_enum_unsigned_64_checked_variants_true_some_kernelversion_new_6_0_0_https_github_com_torvalds_linux_commit_6089fb3_0xccccccccdddddddd_expects
    tests::btf_relocations::relocation_tests::_enum_unsigned_64_true_some_kernelversion_new_6_0_0_https_github_com_torvalds_linux_commit_6089fb3_0xccccccccdddddddd_expects

Presumably my changes have no impact on these, but I cannot tell since the tests failed on the main branch as well.

[gth828r]

I should also mention that I did not test this change on MacOS or Windows, nor do I have a good way to do that right now. I mention that in case this feature was originally left out for some type of OS compatibility reasons.