Add team.externalTeams.list API support

[seratch]

This pull request adds team.externalTeams.list API to this SDK: https://api.slack.com/methods/team.externalTeams.list

Category (place an x in each of the [ ])

• [ ] bolt (Bolt for Java)
• [ ] bolt-{sub modules} (Bolt for Java - optional modules)
• [x] slack-api-client (Slack API Clients)
• [ ] slack-api-model (Slack API Data Models)
• [ ] *slack-api--kotlin-extension** (Kotlin Extensions for Slack API Clients)
• [ ] slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.03%. Comparing base (56f2d26) to head (a889e7f).

Files	Patch %	Lines
...java/com/slack/api/methods/RequestFormBuilder.java	60.00%	2 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1327 +/- ## ========================================= Coverage 75.02% 75.03% - Complexity 4172 4178 +6 ========================================= Files 446 446 Lines 12887 12902 +15 Branches 1329 1331 +2 ========================================= + Hits 9669 9681 +12 - Misses 2443 2444 +1 - Partials 775 777 +2 ```

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