Add BPF token support for program and map loading

[javiercardona-work]

Add support for loading BPF programs and creating maps using BPF tokens,
enabling unprivileged BPF operations in user namespaces with delegated
capabilities.

Changes:

• Add TokenFD field to ProgramOptions and MapOptions
• When set, pass token to kernel with BPF_F_TOKEN_FD flag during syscalls
• Add internal/token package for global token storage
• Add features.SetGlobalToken()/GetGlobalToken() for feature probe support

Since the original PR, the following commits were added:

• Thread BPF token through feature probes: Pass the global token to
  all feature probes (program type, map type, BTF, link, and program
  helper probes) so that capability detection works correctly inside user
  namespaces with delegated BPF permissions.
• Add BPF token integration test program: Standalone test
  (examples/token_test/) that creates a BPF token, optionally drops
  CAP_BPF/CAP_SYS_ADMIN/CAP_PERFMON, and exercises feature probes,
  program loading, and map creation. Validated on a container with
  bpf_token delegation (kernel 6.16.1, unprivileged_bpf_disabled=1).

This is the output of the test program when run on a container with delegated BPF token enabled:

$ /tmp/token-test --drop-caps /run/tw/bpf
Creating BPF token from /run/tw/bpf...
BPF token created: fd=4

Dropping CAP_BPF, CAP_SYS_ADMIN, CAP_PERFMON...
Capabilities dropped. BPF access now depends solely on token.

=== Feature Probes (with token) ===
prog SocketFilter         supported
prog Kprobe               supported
prog SchedCLS             supported
prog XDP                  supported
prog TracePoint           supported
prog CGroupSKB            supported
map  Hash                 supported
map  Array                supported
map  PerfEventArray       supported
map  LRUHash              supported
map  RingBuf              supported

=== Program Load (with token) ===
program load: OK (fd=3)

=== Map Create (with token) ===
map create: OK (fd=3)

=== Without Token (uncached probe) ===
prog PerfEvent (no token):     detect support for PerfEvent: load program: operation not permitted
map  PerCPUHash (no token):     detect support for PerCPUHash: operation not permitted

Done.

[javiercardona-work]

Closing this PR as it is has been superseded by #1953