This is #18266 on v1.3-ossivalis, plus fix from #18289.
This is a stripped down version of #18230, that adds support for --autoloading none|available|all to unittester, without removing support for previous mode (via env variable LOCAL_EXTENSION_REPO). Keeping both possibility in place is somehow less nice, but has the big advantage that can disentangle fixing issues connected to rolling-in this in CI and allows for easier backport of this if deemed useful.

After this PR, the following modes are available for running unittester:

--autoloading none or `` (this is the default)

Same as current default, only statically linked extensions are ever used.
require in test will run a test only if the needed extension is statically linked-in.

LOCAL_EXTENSION_REPO=path/to/repo

This stays as now.
LOCAL_EXTENSION_REPO points to the repository folder.
require in tests will run a test only if the needed extension is statically linked-in OR if the extension is autoloadable
No checks are performed, so setting LOCAL_EXTENSION_REPO will fail tests at runtime where a given extension is not present on the local machine.

--autoloading available

This is similar to the previous, but:
If defined LOCAL_EXTENSION_REPO points to the repository folder, otherwise build/$BUILD_MODE/repository will be used (the default where builds place extensions)
require in tests will run a test only if:

• the needed extension is statically linked-in
• OR if the extension is autoloadable and available (in this case INSTALL will be performed on require, LOAD via autoloading)
• OR if the extension is not autoloadable and available (in this case both INSTALL and LOAD to be performed on require)

--autoloading all

This is similar to the previous with regard to LOCAL_EXTENSION_REPO
require in tests will run a test always:

• if the extension is autoloadable, both INSTALL and LOAD to happen at runtime
• if the extension is not autoloadable, both INSTALL and LOAD to be performed on require
  This mode require either to have a complete set of extensions or pre-filtering the tests to be executed, since require aws with aws extension not available will be a test failure.

An interesting usage for --autoloading all is paired with setting LOCAL_EXTENSION_REPO=http://extensions.duckdb.org, since this will correspond to use the remote available extensions (if there).

Limitation of current PR:

• information about whether a PR is statically linked in is implied from the fact that a given extension can be installed, and NOT from the list of statically linked in extensions. This can be improved on.
• this is currently not run on CI due to 3 issues:
  • run_tests_one_by_one.py needs to be adapted to take more arguments
  • some vss and spatial tests needs to be adapted due to missing requires
  • this allow easier to backport this PR, if one would find that handy