build loadable ext

[wangfenjin]

Address #148

Build example:
cargo build --example hello-ext --features="vtab bundled"

There is one issue when load in duckdb cli:

./build/debug/duckdb -unsigned

D load "/path/to/libhello_ext.dylib";
/path/to/duckdb/src/catalog/catalog_entry/table_function_catalog_entry.cpp:8:79: runtime error: member access within address 0x7ff7bbe317c8 which does not point to an object of type 'duckdb::CreateFunctionInfo'
0x7ff7bbe317c8: note: object is of type 'duckdb::CreateTableFunctionInfo'
 00 00 00 00  58 63 8e 29 01 00 00 00  19 96 04 00 10 61 00 00  00 00 e3 bb f7 7f 00 00  20 17 e3 bb
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'duckdb::CreateTableFunctionInfo'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /path/to/duckdb/src/catalog/catalog_entry/table_function_catalog_entry.cpp:8:79 in

Seems related to https://stackoverflow.com/a/57304113/1203241 , but haven't figure out the solution.

From https://github.com/Mause/duckdb-deltatable-extension/blob/main/CMakeLists.txt , seems by using build_loadable_extension may work, but not sure if we have a "cleaner" way to achieve this.

@Mause Any suggestion?

[wangfenjin]

If I download the cli from duckdb release page, it works

[codecov]

Codecov Report

Merging #149 (a3f454e) into main (80a492c) will decrease coverage by 0.27%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #149      +/-   ##
==========================================
- Coverage   58.03%   57.76%   -0.27%     
==========================================
  Files          34       34              
  Lines        2035     2041       +6     
==========================================
- Hits         1181     1179       -2     
- Misses        854      862       +8     

Impacted Files	Coverage Δ
src/lib.rs	72.54% <66.66%> (-4.54%)	⬇️

... and 3 files with indirect coverage changes

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