feat(shield): allow custom env vars and volume mounts

[aroberts87]

What this PR does / why we need it:

Allow the users to specify custom environment variables and volumes/volume mounts into the Cluster Shield and Host Shield pods/containers.

Checklist

• Title of the PR starts with type and scope, (e.g. feat(agent,node-analyzer,sysdig-deploy):)
• Chart Version bumped for the respective charts
• Variables are documented in the README.md (or README.tpl in some charts)
• Check GithubAction checks (like lint) to avoid merge-check stoppers
• All test files are added in the tests folder of their respective chart and have a "_test" suffix

[aroberts87]

@AlbertoBarba - 7f0a878 was added purely to bypass the helm upgrade issue caused by the use of --reuse-values flag in the CI flows. Realistically this is not a situation we'll have to guard against at the moment due to the chart still being under heavy development. We can remove these in a follow up commit to reduce the cruft in the templates.