Align gitlab_group and gitlab_groups data source models

Extract the shared per-group schema and Go model into schema_gitlab_group.go so the singular gitlab_group and plural gitlab_groups data sources expose identical attribute names, types, and shapes for the group body. The singular flow keeps its discriminator overrides (group_id, full_path) and the framework id; the plural flow re-declares group_id and full_path as plain Computed attributes for its nested item shape. No new upstream fields are mapped in this commit.

Changelog: IMPROVEMENTS Subsystem: datasource/gitlab_group

Add missing fields avatar_url, repository_storage, max_artifacts_size, default_branch_protection_defaults, file_template_project_id, share_with_group_lock, require_two_factor_authentication, two_factor_grace_period, project_creation_level, subgroup_creation_level, auto_devops_enabled, emails_enabled, emails_disabled (deprecated), mentions_disabled, ldap_cn, ldap_access, ip_restriction_ranges, allowed_email_domains_list, only_allow_merge_if_pipeline_succeeds, allow_merge_on_skipped_pipeline, only_allow_merge_if_all_discussions_are_resolved, created_at, marked_for_deletion_on, statistics, custom_attributes.