feat: implement pull secrets in helm chart

[mars64]

Supports the use of imagePullSecrets in deployment and serviceAccount objects. Useful for things like authenticating to DockerHub in systems that would otherwise hit pull limits.

This would close #191

Please note: I bypassed the pre-commit hooks as I did not change any of the Java code and am not currently set up with the required local Gradle toolchain. Apologies in advance.

[mars64]

For context, it appears that this repo's CI actions are hitting the same issue I had with the pre-commit hooks. See https://github.com/AbdelrhmanHamouda/locust-k8s-operator/actions/runs/8990973580 for more details.

[AbdelrhmanHamouda]

Thank you for your contribution to the project and thanks for your patience till i answered this request.

It is true that it seems the actions are hitting an issue and it is not related to you.

I have reviewed your change and I'm accepting it.

Thank you again for the contribution, it is always appreciated :)
I'll work between today and tomorrow to release a new version with your code changes included