adds a default clearml-core service account for the core components

[thoraxe]

What this PR does / why we need it:
This PR adds a default service account declaration to the clearml components (webserver, fileserver, apiserver) and a YAML to the chart to create that SA.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Verify the work you plan to merge addresses an existing issue (If not, open a new one) (required)
• Check your branch with helm lint (required)
• Update version in Chart.yaml according semver rules (required)
• Substitute annotations section in Chart.yaml annotating implementations (useful for Artifecthub changelog) (required)
• Update chart README using helm-docs (required)

Which issue(s) this PR fixes:
Fixes #247

Special notes for your reviewer:
It's not entirely a complete PR because it should either do one of two things, which I'm not entirely sure how to do:

1. enable or disable using the ServiceAccount at all
2. create individual SAs for each component based on the name specified in values.yaml

[valeriano-manassero]

README needs to be synced

[valeriano-manassero]

@thoraxe I did some changes to reach CI ok status.
Would you mind to check just to be sure this is what you needed in this PR?

[thoraxe]

It looks like it does the job to me -- it adds a default service account for the core components with a value. Thanks for cleaning up my mess!