fix(cp-schema-registry): Support creating secrets and volumes

[joshuasimon-taulia]

This is basically copy/pasting #389 and applying it to schema-registry

What changes were proposed in this pull request?

• This PR allows creating Kubernetes secret and mounting volumes
• Explanatory comments have been added in the values file for the newly introduced values

How was this patch tested?

• helm template locally
• kubectl apply against a v1.19.9-gke.1400 cluster
• the testing done against previous PR

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[joshuasimon-taulia]

[clabot:check]

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@confluentinc It looks like @joshuasimon-taulia just signed our Contributor License Agreement. :+1:

Always at your service,

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[joshuasimon-taulia]

i've been running these changes since June 2 without any issues. @SoerenHenning @OuesFa @kppullin anyone with merge access around?

[joshuasimon-taulia]

@gAmUssA mind taking a look?