Fix operator classes for lantern_hnsw access method

[var77]

Currently there's an issue that when pgvector exists and we create lantern_hnsw access method, operator classes are not being registered on that access method, but on hnsw, so we are unable to create Lantern hnsw index.
This change will make sure to create operator classes on right access method. I have created a function _create_ldb_operator_classes to avoid code duplication.

[github-actions]

Benchmarks

metric	old	new	pct change
recall (after create)	0.740	0.740	-
recall (after insert)	0.776	0.774	-0.26%
select bulk tps	234.291	223.460	-4.62%
select bulk latency (ms)	31.739	34.754	+9.50%
select bulk latency (stddev ms)	11.093	8.409	-24.20%
create latency (ms)	1447.931	1469.768	+1.51%
insert bulk tps	14.880	14.516	-2.45%
insert bulk latency (ms)	67.187	68.877	+2.52%
insert bulk latency (stddev ms)	2.692	2.561	-4.87%
disk usage (bytes)	6348800.000	6348800.000	-

[codecov]

Codecov Report

Merging #180 (3b54578) into main (4c80eca) will increase coverage by 0.16%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
+ Coverage   82.93%   83.09%   +0.16%     
==========================================
  Files          17       17              
  Lines        1201     1201              
  Branches      256      256              
==========================================
+ Hits          996      998       +2     
+ Misses         87       81       -6     
- Partials      118      122       +4     

see 2 files with indirect coverage changes