refactor(diskann): decouple from libaio via dlopen

[richyreachy]

Currently, DiskAnn ships as a separate runtime-loaded .so (libzvec_diskann_plugin.so) file which requires a hard build-time dependency on libaio-dev.

This PR improves with a simpler, more robust design. Probing libaio.so file, a thread-safe singleton dlopens it and caches the syscall pointers. If libaio is absent, it will fall back to degrade to synchronous pread() with a warning.

The following shows the performance comparison on Cohere 1M between pread w/ and w/o AIO enabled. (Aliyun g9i, PL0, 10000 IOPS)

Pread w/ AIO Enabled

Result	FP32-100	FP32-300	FP32-500	FP16-100	FP16-300	FP16-500
Search Mem（4 thread）	503M	503M	503M	486M	486M	486M
Recall@1	96.1%	98.9%	99%	95.3%	97.8%	98.4%
Recall@10	96.14%	98.6%	98.94%	95.86%	98.1%	98.63%
Recall@50	93.394%	97.782%	98.598%	93.452%	97.354%	98.188%
QPS（1 thread）	32.6	11.2	6.7	32.3	11.1	6.7
QPS（2 thread）	65.2	22.3	13.5	64.5	22.4	13.4
QPS（4 thread）	106.4	36.6	22.1	106.4	36.6	22.1

Pread w/o AIO Enabled

Result	FP32-100	FP32-300	FP32-500	FP16-100	FP16-300	FP16-500
Search Mem（4 thread）	503M	503M	503M	486M	486M	486M
Recall@1	96.1%	98.9%	99%	95.3%	97.8%	98.4%
Recall@10	96.14%	98.6%	98.94%	95.86%	98.1%	98.63%
Recall@50	93.394%	97.782%	98.598%	93.452%	97.354%	98.188%
QPS（1 thread）	18.4	6.3	3.8	18.3	6.3	3.8
QPS（2 thread）	36.8	12.6	7.5	36.5	12.6	7.6
QPS（4 thread）	73.1	25.2	15.2	72.5	25.1	15.2

[iaojnh]

可以把workflow里，打包相关的libaio-devel依赖，在这个PR里移除

[egolearner]

LGTM