INTMDB-933: Make region atributed optional computed in third-party-integration

[maastha]

Description

When configuring PAGER_DUTY integration, the user may/may not set region attribute. If user doesn't set it, the API returns a default value during READ, currently because of that every time the user runs terraform plan, the 'region' attribute is determined as modified.
This PR therefor marks the 'region' attributed as Optional and Computed.

Link to any related issue(s): INTMDB-933
#1316

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[andreaangiolillo]

LGTM