[Fleet] Add elastic-api-version support

[nchaulet]

Description

Introduce a new middleware that support elastic-apiversion header to allow versioning of the Fleet server APIs.

If the header is not provided we fallback to a default version.

For PR readability I will add some integration tests that duplicate the client and the api tests suite in a following PR.

Todo

• Unit tests the version middleware

[elasticmachine]

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• Start Time: 2023-06-12T13:31:43.621+0000

• Duration: 38 min 16 sec

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Total	717

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[nchaulet]

I am wondering if there is a better place to list the versions we support and the default version.

[michel-laterman]

think it looks good, mostly have nitpicks.

We may want to consider altering the openapi spec to show responses will contain the version header (we don't do this for request ids, but we should)

The only real concern I have is now we're changing the behaviour of the status endpoint as well

[nchaulet]

The only real concern I have is now we're changing the behaviour of the status endpoint as well

in my opinion is probably okay as if you do not provide the Elastic-Api-Version it will still work

[mergify]

This pull request is now in conflicts. Could you fix it @nchaulet? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b feature-api-version upstream/feature-api-version
git merge upstream/main
git push upstream feature-api-version