[stable/locust]: Add additional custom manifests

[langesven]

Description

This adds an easy way to deploy additional manifests alongside the locust chart like for example external secrets that can then be mounted into the locust pods via existing means (environment_external_secret)

Checklist

• [x] Title of the PR starts with chart name (e.g. [stable/mychartname])
• [x] I have read the contribution instructions, bumped chart version and regenerated the docs
• [ ] Github actions are passing