feat(bpf/openssl): Add SSL pointer to FD cache as fallback

[jonfriesen]

Introduces an LRU hash map (ssl_to_fd_map) to cache the mapping between an SSL structure pointer and its associated file descriptor (FD).

Problem:
The get_fd function, when called from SSL uprobe return handlers (uretprobes), sometimes failed to retrieve the FD. This was determined to be a timing issue where the uretprobe could execute before the backing I/O syscall's exit tracepoint had run to update the uprobe_fd_requests map via respond_to_fd_request. This resulted in missed data events for SSL traffic.

Solution:
This change implements a caching layer as a fallback:

• A new LRU map ssl_to_fd_map stores uintptr_t ssl -> int32_t fd.
• The map is populated within the SSL_read/write uretprobes after get_fd successfully retrieves an FD (using the existing primary ssl_get_fd or secondary get_fd_from_syscall methods).
• A new function get_fd_from_cache looks up the SSL pointer in this map.
• get_fd is modified to attempt get_fd_from_cache as a third resort if the primary and secondary methods fail.
• The SSL_free uprobe is updated to delete the corresponding entry from ssl_to_fd_map.

Future:
We may want to consider hooking into the the SSL_set_fd, SSL_set_rfd, and SSL_set_wfd to populate the cache by pointer. Though it seems largely unnecessary now.