add aws-iso and aws-iso-b partition

[kaiyan-sheng]

This PR adds aws-iso and aws-iso-b into the supported AWS partition list. Also added GetEndpoint function to assign DNS suffix endpoint based on the given AWS partition names.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

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Build stats

• Build Cause: Pull request #23209 updated

• Start Time: 2020-12-18T01:30:22.207+0000

• Duration: 53 min 14 sec

Test stats 🧪

Test	Results
Failed	0
Passed	5457
Skipped	380
Total	5837

Steps errors

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Terraform Apply on x-pack/metricbeat/module/aws

• Took 0 min 14 sec . View more details on here
• Description: terraform apply -auto-approve

Terraform Apply on x-pack/metricbeat/module/aws

• Took 0 min 15 sec . View more details on here
• Description: terraform apply -auto-approve

💚 Flaky test report

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Test stats 🧪

Test	Results
Failed	0
Passed	5457
Skipped	380
Total	5837

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[kaiyan-sheng]

We would like to remove all hardcoded switch cases. Will close this PR.