docs: add snippet and test for a Firestore client with a regional endpoint

[jlara310]

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Fixes #1255 ☕️

If you write sample code, please follow the samples format.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/com/example/firestore/snippets/RegionalEndpointSnippets.java:19, tag firestore_regional_endpoint

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[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• samples/install-without-bom/pom.xml
• samples/snapshot/pom.xml
• samples/snippets/pom.xml

[jlara310]

@kolea2 , PTAL

[jlara310]

@telpirion , could you review this one, too?

[jlara310]

@MarkDuckworth , could you help review this code snippet?