OpenAPI: add Lambda that maintains the spec version index

[reakaleek]

What

• Add a Lambda that maintains index.json at the root of the elastic-docs-openapi-specs bucket. The index resolves each published major version of an OpenAPI spec to its highest minor, and keeps main as its own entry.
• Add Elastic.Documentation.OpenApiIndex, a shared library that holds the index format, the build logic, and the S3 publish logic, with unit tests.

Why

• Specs upload to org/repo/version/file. Today nothing maps a major version to the newest spec published for it, so a consumer must list the bucket and pick a winner itself.
• The index gives docs-builder a single stable file to read for that answer.
• Implements https://github.com/elastic/docs-eng-team/issues/714, which https://github.com/elastic/docs-eng-team/issues/718 depends on.

Notes

• Each event rebuilds the whole index from a full bucket listing instead of merging the triggering event. The rebuild self-heals after a missed event, and a deleted object simply cannot produce an entry.
• The write uses an S3 ETag precondition. If a concurrent invocation wins, the message returns to the queue and SQS redelivery repeats the rebuild. There is no in-process retry.
• The main or <major>.<minor> version contract is enforced upstream in elastic/docs-actions openapi/upload, which ships separately. The Lambda logs and skips any key that does not match that shape.

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