Add support for USDT probes

[nbaksalyar]

As discussed in #307, there should be public API to use USDT probes.

The attach_usdt signature can mirror the uprobe API (because internally USDTs are simply uprobes with some extra features and publicly available metadata):

pub fn attach_usdt(
  &mut self,
  probe_name: &str,
  offset: u64,
  target: &str,
  pid: Option<pid_t>,
)

This function should read probe addresses from the target ELF header (similarly to how it's done in redbpf::symbols::ElfSymbols).

We will also needs a function to enable these probes -- either automatically or manually -- as many USDT probes require to update dynamic reference counters (called semaphores) to work properly.