Validators cause unwanted fetch of file URL

# Description

When the WPS execution occurs and that an input reference URL (for example remote JSON for below references), the format validator will actually pull the file because `file` property refers to `UrlHandler.file`, which in turn does the request and write the chucks locally.

https://github.com/geopython/pywps/blob/d05483d75e753b3cda303f5c0bb778a0f9465393/pywps/validator/complexvalidator.py#L126

https://github.com/geopython/pywps/blob/d05483d75e753b3cda303f5c0bb778a0f9465393/pywps/validator/complexvalidator.py#L151

In `UrlHandler.file` :
https://github.com/geopython/pywps/blob/d05483d75e753b3cda303f5c0bb778a0f9465393/pywps/inout/basic.py#L461-L467

This might be *ok for usual data processing execution* because most processes want the file to be generated locally at some point, but it shouldn't be done during validation when `MODE < STRICT` as it is not required for checking the extension from the name (which is what `MODE.SIMPLE` attempts to do).
The behavior is valid when `MODE >= STRICT` because the full contents are validated, but it is not required for simple mime-type checks.

My general use case is that I need to reference `https://` files and pass them down to further remote processes. Therefore, I want to validate that the file type is correct, but not fetch them right away (the child process will do so).
As the file could be quite big, fetching it 2 times (parent/child process) for basic validation is not great. Since the file is not required locally until I explicitly call `file` property, the validator shouldn't do so if it doesn't need it.

# Environment

- operating system:
- Python version:
- PyWPS version: 4.2.4
- source/distribution
- [x] git clone
- [ ] Debian
- [ ] PyPI
- [ ] zip/tar.gz
- [ ] other (please specify):
- web server
- [ ] Apache/mod_wsgi
- [ ] CGI
- [ ] other (please specify):

# Steps to Reproduce

- Setup a WPS with an `ComplexInput` and some format with `MODE.SIMPLE`.
- Execute the WPS process and see that the file is fetched before even arriving in the handler.

# Additional Information

Part of requirements for developing OGC EMS which dispatches execution to remote ADES.
https://github.com/crim-ca/weaver