chore(concepts/termination): split samples into directories

[ace-n]

Example PR for b/230368998

Questions for reviewers:

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If we want to switch from functions_ to cloudfunctions_ as a region-tag prefix, should I do that:

1. in this PR?
2. as part of a separate effort?

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These samples aren't really practical to test. I assume, therefore, the lack of tests is OK?

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 2 region tags.

• functions/concepts/backgroundTermination/index.js:20, tag functions_concepts_node_termination
• functions/concepts/httpTermination/index.js:20, tag functions_concepts_node_termination_http

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[iennae]

separate effort as the functions -> cloudfunctions is a large sized effort and involves collaboration with TW