Provide option for west init or west update to fetch all submanifests

[aescolar]

It would be nice having an option/command line switch in west init that would make it fetch all repos which have sub-manifests (projects with import) at init time.
Alternatively it could be a new command line switch to west update which instructs it to fetch those same repos (and nothing else).
Either of these would not (need to) find projects with sub-manifests that are affected by group filters or allow-lists (filtered out).

The aim would be to be able to have a workspace in which west can already work without erroring out, but without the need to pull all repositories content.

An example use-case is the solution to the problem described here:
zephyrproject-rtos/zephyr#55696 (comment)
in which after west init, as the main manifest has a project with a sub-manifest/import, west errors out if one tries to use quite a bit of its functionality (for ex. anything related to manifest, list or status).
A solution to this particular problem, is to either do a full west update pulling all projects (which may be a quite a bit of unnecessary overhead), OR, to do a west update <missing_repos_with_imports> which requires knowledge of which repos are the ones with sub-manifests.

[aescolar]

CC @stephanosio

[tomi-font]

This issue seems somewhat related to a problem I faced (and is one of the issues I stumbled upon in my search for solution), so commenting here.

Basically my problem was that I needed the tf-m-tests repo (which is defined in submanifests/optional.yaml) to be updated to the manifest revision because I had changed it and was building a sample that made use of it.
Because it was somehow already present (I don't remember myself doing anything to fetch those repos) I assumed that the simple west update did update those repos as well. But turns out not, and this did cost some time. (At least the documentation?)

I eventually found the command that does the job I needed: west update --gf +optional
But it was hard to find.
I think some thing(s) could be improved with regards to this, but I don't quite know what exactly. At least the documentation?
(edit: Just found out about west config manifest.group-filter +optional. With this a simple west update is enough.)

[marc-hb]

west list | grep tf-m ?

[tomi-font]

With west config manifest.group-filter +optional:

psa-arch-tests modules/tee/tf-m/psa-arch-tests 2cadb02a72eacda7042505dcbdd492371e8ce024 https://github.com/zephyrproject-rtos/psa-arch-tests
tf-m-tests modules/tee/tf-m/tf-m-tests 08a3158f0623a4205608a52d880b17ae394e31d2 https://github.com/zephyrproject-rtos/tf-m-tests
trusted-firmware-m modules/tee/tf-m/trusted-firmware-m 0b898c9b72171b0a260c0bb64a92ea4713f9e931 https://github.com/zephyrproject-rtos/trusted-firmware-m

With west config manifest.group-filter '':

trusted-firmware-m modules/tee/tf-m/trusted-firmware-m 0b898c9b72171b0a260c0bb64a92ea4713f9e931 https://github.com/zephyrproject-rtos/trusted-firmware-m