feat: add postgresql_role_attribute to manage role attributes

[michaellzc]

ref cyrilgdn#210

use cases:

When a role is managed externally, e.g., google_sql_user, and such provider does not provide ability to customize role attribute. postgresql provider that has direct connection to the db host can be used to perform such automation.

this PR introduced a new postgresql_role_attribute resource to manage role attribute without holding on to the role itself.

design decisions:

• it's non-authoritative, unlike postgresql_role. it does not assume any default value for the attribute, and ignore any nil attribute.
• on destroy, it will simply orphaned the attribute without attempting to revert it in any way. this is intentional to avoid the headache of conflicted resources.

[michaellzc]

• feat/postgresql_role_attribute: add support for extension GUC settings #3
• feat: add postgresql_role_attribute to manage role attributes #2 👈 (View in Graphite)
• main

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