Backport of Plugins: Auto version selection for auth/secrets + tune version into release/1.12.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #17167 to be assessed for backporting due to the inclusion of the label backport/1.12.x.

The below text is copied from the body of the original PR.

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Two main features:

• Auto version selection for auth/secret engines when versioned plugins are registered
• Add the ability to tune plugin version for auth/secret engines so that in-place upgrades can be performed

Other changes:

• external_plugin_test.go moves the calling code a level up the stack, using the system backend's HandleRequest instead of directly calling the implementation functions for each route. Moves the tests slightly closer to production call stacks.

There's still some more follow-up work to do in future PRs, including:

• Website documentation
• Add -plugin-version flag to vault auth tune and vault secrets tune
• Replace version with plugin_version everywhere (I excluded that from this PR to avoid diff noise) Done in Plugins: Consistently use plugin_version #17171
• Take a holistic look at the changelogs for this set of features

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Overview of commits

• e63bf37