feat: add version_file input to read the gcloud version from a file

[somaz94]

Adds an optional version_file input that resolves the Cloud SDK version from a file, mirroring the *-version-file pattern in actions/setup-go, actions/setup-node, and actions/setup-python.

Two formats are supported:

• a plain version file containing a single version string or constraint (e.g. 416.0.0 or >= 416.0.0)
• an asdf/mise .tool-versions file containing a gcloud <version> line

Precedence: an explicitly set version (anything other than the default latest) wins and version_file is ignored with a warning. The version-resolution logic is unchanged once a concrete version is determined, so the existing install/cache path is reused as-is.

Validation (run locally):

• npm run lint (eslint) passes
• npm test passes, 22/22 (added 3 run() cases for .tool-versions / plain file / version precedence, plus 5 parseVersionFile unit cases)
• npm run build (ncc) rebuilt dist/index.js
• npm run docs regenerated the README inputs section

[google-cla]

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[somaz94]

Friendly ping — this has been green and unreviewed for about a month.

One clarification in case the bot comment above caused confusion: the CLA is signed — the cla/google check passes on the current head. The 2026-06-24 "you will need to sign" comment is stale.

The change adds a version_file input so the gcloud version can be read from a file instead of being pinned inline. Happy to adjust if the input name or lookup order should differ.

[somaz94]

Closing this — it has been open since June with no review, and I would rather not leave a stale PR sitting in the queue. Happy to reopen if the version_file input is something you would like.