fix: Added missing attribute source_countries to ZIA zia_firewall_filtering_rule

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Description

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Types of changes

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[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[x] My code follows the code style of this project.
[x] My change requires a change to the documentation.
[x] I have updated the documentation accordingly.
[x] I have added tests to cover my changes.
[x] All new and existing tests passed.
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Codecov Report

Attention: Patch coverage is 11.11111% with 16 lines in your changes missing coverage. Please review.

Project coverage is 72.05%. Comparing base (45a003a) to head (d0190b3).

Files with missing lines	Patch %	Lines
zia/utils.go	0.00%	8 Missing :warning:
zia/resource_zia_firewall_filtering_rules.go	0.00%	7 Missing :warning:
zia/validator.go	0.00%	1 Missing :warning:

Additional details and impacted files

[![Impacted file tree graph](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375/graphs/tree.svg?width=650&height=150&src=pr&token=A9J4AJS7F5&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler)](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler) ```diff @@ Coverage Diff @@ ## master #375 +/- ## ========================================== - Coverage 72.25% 72.05% -0.20% ========================================== Files 82 82 Lines 13910 13918 +8 ========================================== - Hits 10051 10029 -22 - Misses 3117 3152 +35 + Partials 742 737 -5 ``` | [Files with missing lines](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler) | Coverage Δ | | |---|---|---| | [zia/common.go](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375?src=pr&el=tree&filepath=zia%2Fcommon.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler#diff-emlhL2NvbW1vbi5nbw==) | `82.30% <100.00%> (ø)` | | | [zia/resource\_zia\_forwarding\_control\_rule.go](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375?src=pr&el=tree&filepath=zia%2Fresource_zia_forwarding_control_rule.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler#diff-emlhL3Jlc291cmNlX3ppYV9mb3J3YXJkaW5nX2NvbnRyb2xfcnVsZS5nbw==) | `41.53% <ø> (-8.47%)` | :arrow_down: | | [...resource\_zia\_fw\_filtering\_ip\_destination\_groups.go](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375?src=pr&el=tree&filepath=zia%2Fresource_zia_fw_filtering_ip_destination_groups.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler#diff-emlhL3Jlc291cmNlX3ppYV9md19maWx0ZXJpbmdfaXBfZGVzdGluYXRpb25fZ3JvdXBzLmdv) | `64.33% <ø> (+1.39%)` | :arrow_up: | | [zia/validator.go](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375?src=pr&el=tree&filepath=zia%2Fvalidator.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler#diff-emlhL3ZhbGlkYXRvci5nbw==) | `33.09% <0.00%> (ø)` | | | [zia/resource\_zia\_firewall\_filtering\_rules.go](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375?src=pr&el=tree&filepath=zia%2Fresource_zia_firewall_filtering_rules.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler#diff-emlhL3Jlc291cmNlX3ppYV9maXJld2FsbF9maWx0ZXJpbmdfcnVsZXMuZ28=) | `3.85% <0.00%> (ø)` | | | [zia/utils.go](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375?src=pr&el=tree&filepath=zia%2Futils.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler#diff-emlhL3V0aWxzLmdv) | `43.47% <0.00%> (-1.98%)` | :arrow_down: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/zscaler/terraform-provider-zia/pull/375/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zscaler)

zscaler / terraform-provider-zia