Rate limit the DAST configuration page

What does this MR do and why?

Adds a rate limit to the DAST configuration page (Projects::Security::DastConfigurationController#show). Each view expands the project's CI config, including remote includes, in the web request, and nothing limits how often a user can do that.

Behind the dast_configuration_rate_limit feature flag (disabled by default):

• Rate limit. dast_configuration_show: 10 views per minute per user, counted across all of the user's projects. Requests over the limit return 429 before the CI config is read.

The limit is a fixed value. It's explained in https://gitlab.com/gitlab-org/gitlab/-/work_items/637251#note_4197362844. It's documented as a non-configurable rate limit.

Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/637251

How to set up and validate locally

1. Enable the flag: Feature.enable(:dast_configuration_rate_limit).
2. As a Developer on an Ultimate project, open Secure > Security configuration and select Configure DAST. The page loads as normal.
3. Reload the page 11 times within a minute. The 11th returns 429.

MR acceptance checklist

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