Add support and tests for install-time schema relocation

[Ngalstyan4]

Our extension cannot be fully relocatable since its objects span multiple schemas (we have _lantern_internal schema) It can be (and as of this commit) is install-time relocatable so it can be installed on a custom schema, different from the default one. See 38.17.2 in the link below for details
https://www.postgresql.org/docs/current/extend-extensions.html#EXTEND-EXTENSIONS-RELOCATION

Note: access methods in postgres are not schema-qualified so they are always available globally, regardless of the schema on which the extension is installed
https://www.postgresql.org/docs/current/sql-create-access-method.html

[github-actions]

Benchmarks

metric	old	new	pct change
recall (after create)	0.740	0.740	-
recall (after insert)	0.746	0.758	+1.61%
select bulk tps	485.043	480.111	-1.02%
select bulk latency (ms)	15.715	15.980	+1.69%
select bulk latency (stddev ms)	2.945	2.403	-18.40%
create latency (ms)	1219.584	1217.568	-0.17%
insert bulk tps	11.099	11.400	+2.71%
insert bulk latency (ms)	90.092	87.713	-2.64%
insert bulk latency (stddev ms)	1.482	3.544	+139.14%
disk usage (bytes)	6348800.000	6348800.000	-

[codecov]

Codecov Report

Merging #206 (1b70b5c) into main (3474f82) will increase coverage by 0.08%.
Report is 7 commits behind head on main.
The diff coverage is 85.71%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #206      +/-   ##
==========================================
+ Coverage   83.05%   83.14%   +0.08%     
==========================================
  Files          18       18              
  Lines        1216     1234      +18     
  Branches      261      264       +3     
==========================================
+ Hits         1010     1026      +16     
- Misses         81       83       +2     
  Partials      125      125              

Files	Coverage Δ
src/hnsw/external_index.c	91.12% <100.00%> (+0.03%)	⬆️
src/hnsw/insert.c	82.53% <100.00%> (+0.28%)	⬆️
src/hnsw/scan.c	81.37% <100.00%> (+0.37%)	⬆️
src/hnsw/utils.h	40.00% <ø> (ø)
src/hnsw/utils.c	88.00% <90.90%> (+2.28%)	⬆️
src/hnsw/build.c	80.32% <60.00%> (-0.79%)	⬇️

... and 1 file with indirect coverage changes