feat: optional Watson assistant access policies

[in-1911]

Description

• Optionally add access policies targeting Watson assistant resources created by the DA (assistant, skills, environment)
  • The target use case: multiple teams are deploying their stack configuration, using the same Watson Assistant service instance and only will get editor access to the assistant "workspace" that their stack deployment creates.
  • Policies are added to an existing access group named in existing_wa_access_group_name parameter
  • The group is likely to be configured to include other access policies related to the solution deployment, so it's not quite practical to create a new access group at this point
  • Watson resource access roles are set to "Reader", "Writer", "Viewer", "Editor"
  • Uses terraform-ibm-iam-access-group module
• Fix for uppercase characters in prefix that would cause a validation error with Elastic index

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[in-1911]

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[in-1911]

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