#14568 added a mechanism to add certain files and dirs to an allowlist to allow some IO even when enable_external_access is false.

This PR aims to extend that to also allow installing extensions.

With this PR you can:

SET extension_directory= 'my_extension_dir';
SET allowed_directories=['my_extension_dir', 'http://', 'https://'];
SET enable_external_access=false;
install my_ext;

Note that when using absolute paths EITHER the extension directory needs to exists already OR the allowed directories should contain the path to the parent dir of the extension dir. This is because to create the extension directory, FileSystem::DirectoryExists needs to be called on the directory in which the extension directory will sit. So for example:

SET extension_directory= '/some/path/to/extensions';
where /some/path/to/extensions does not exists yet will only work with:

SET allowed_directories=['/some/path/to', 'http://', 'https://'];