Support team IDs when listing users

[alloutflo]

Summary

• add the optional team_id field to UsersListParams, serialized in Slack's expected snake_case form
• add a backward-compatible listUsers(String teamId) API and implement it in SlackWebClient
• retain team_id while following cursor pagination
• cover both parameter serialization and the actual form-encoded users.list request

Fixes #353.

Testing

• mvn -B -q clean install under Temurin 21: 300 tests passed, 0 skipped
• mvn -B -q spotless:check
• git diff --check

The request path is exercised without live network access; no Slack org token is required by the regression test.

AI assistance disclosure

OpenAI Codex was used to analyze, implement, and test this change. The diff and checks listed above were verified in the contribution workspace.

[alloutflo]

@opaliukh Maintainer handoff: this PR is mergeable, up to date with master, and has no review comments or open threads. The full Maven build passes locally (300 tests), Spotless passes, and the regression covers both team_id serialization and the actual form-encoded users.list request without a live Slack token.

The CI workflow is action_required with 0 jobs until a maintainer approves it. Could you or another maintainer approve that run and review the backward-compatible API addition?