Add support PEP-740 attestations for GitLab CI/CD

[facutuesca]

Adds support for uploading PEP-740 attestations generated from a GitLab CI/CD workflow.
The actual verification is done inside pypi-attestations, similar to GitHub.

Tested locally with an attestation signed from GitLab CI/CD:

Rekor entry: https://search.sigstore.dev/?logIndex=149980684

Part of #17001

cc @woodruffw @di

[facutuesca]

TODO: Once merged, I'll open a PR adding documentation on how to generate the attestations from a GitLab workflow
docs PR here: #17133

[woodruffw]

LGTM, great work @facutuesca!

[woodruffw]

Flagging one thing (could be done now or in a follow-up): the publisher macro also needs to be updated:

warehouse/warehouse/templates/includes/file-details.html

Lines 15 to 24 in dde8b3d

	{% macro publisher(publ) -%}
	{% if publ.kind == "GitHub" %}
	<p>
	Publisher: <a href="https://github.com/{{ publ.repository }}/blob/HEAD/.github/workflows/{{ publ.workflow }}">
	<i class="fa-brands fa-github" aria-hidden="true"></i>
	<code>{{ publ.workflow }}</code> on {{ publ.repository }}
	</a>
	</p>
	{% endif %}
	{%- endmacro %}

[facutuesca]

Flagging one thing (could be done now or in a follow-up): the publisher macro also needs to be updated:

Good catch, fixed!

[facutuesca]

Opened PR for the docs here: #17133