perf(access): read the token once instead of once per header step

[rohilsurana]

Follow up to #23, which has merged. Rebased onto main, so the diff here is just
this change.

append_claims_as_headers and verify_organization_id_header each decoded the
same token, so a request that used both read it twice. run() now decodes once
and passes the claims to both.

The decode still does not happen at all when nothing needs the claims.
verify_organization_id_header used to read the org header itself and return
early when it was missing, so run() now reads it once and passes the value
down to keep that.

Only the decode moved. Each function keeps its own handling of an unreadable
payload: the claim headers refuse the request, the org check drops the header.
Merging those into one guard would have turned the second into a 401.

Four cases count decodes through a wrapper around the real decoder. On the old
code the first one fails with expected 1 got 2; the other three pass either
way, because the double decode only happened when both steps ran and the client
actually sent the org header.

73 passed on Kong 3.4 and 3.9, up from 69. luacheck unchanged, 0 errors.

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