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