[charts/portal]:Provision to configure custom registries to pull images for Portal

[ksaladi]

Description of the change

Allows configuring custom repositories with appropriate credentials for installing Portal helm charts. Changes contain:

• New variable 'portal.useExistingPullSecret' to use existing pullsecret.
• New section ('portal.imagePullSecret') that contains the enabled,username,password fields to configure custom registries

Benefits

Same as above

Drawbacks

None

Applicable issues

None

Additional information

Checklist

• [x] Chart version bumped in Chart.yaml according to semver.
• [x] Variables are documented in the README.md
• [x] Title of the PR starts with chart name (e.g. [charts/gateway])
• [x] If the chart contains a values-production.yaml apart from values.yaml, ensure that you implement the changes in both files