release-22.1: apiv2: accept cookie auth when header is non-empty

[dhartunian]

Backport 1/1 commits from #84617.

/cc @cockroachdb/release

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In order to make use of HTTP endpoints under /api/v2 in the DB Console
it is necessary to support cookie-based authentication for ergonomic
Javascript use.

Previously, header-based auth was not possible to use in the DB Console
because the login endpoint we use returns the session in a Cookie.
Moving this cookie into a header would require us to read into a
less-secure storage method (local storage, redux, etc.) instead of
keeping it secure in the browser's cookie storage.

We implement a suggestion to rely on Cookie auth by requiring the
presence of the auth header with a magic value of "cookie" that tells the
server to look for the session in the session cookie. This forces the caller
to modify the request via JS, which protects us from CSRF since
cross-origin requests can only be "simple". See the issue for further
discussion.

Resolves #84311

Release note (security update): The HTTP endpoints under the /api/v2 prefix
will now accept cookie-based authentication similar to other HTTP endpoints
used by the DB Console. The encoded session must be in a cookie named
"session", and the "X-Cockroach-API-Session" header is required to be set
to "cookie" for the session to be read from the cookie header. A cookie
provided without the custom header present will be ignored.

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Release justification: additive improvement to the /api/v2 endpoints to allow for ease of use with UI code.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is