Add ST_AsHEXWKB, updated casts, additional fixes.

[Maxxen]

Closes #50
Closes #49
Closes #43
Closes #37
Closes #42

Functions & Casts

• Adds new ST_AsHEXWKB(geometry) to produce WKB in the same hex-format as e.g. postgis.
• Casts between WKB_BLOB and GEOMETRY are now explicit rather than implicit, and alias ST_AsWKB/ST_GeomFromWKB.
• Adds an optional ST_GeomFromText(<wkt>, ignore_invalid := <bool>) keyword parameter to produce nulls when encountering invalid WKT strings.

GDAL Module

• Automatic detection of optional vector drivers are now disabled, all supported drivers are instead explicitly enabled.
• There is now a SPATIAL_USE_NETWORK=<bool> CMake build option to control building/linking curl and openssl, as well as whether or not GDAL drivers requiring network functionality should be enabled. This takes us another step closer towards Feature: WASM Support #17. Enabled by default.