REST spec: Add create/drop/exists function endpoints to OpenAPI spec

[huaxingao]

Follow-up to #15180, which added listFunctions and loadFunction. This adds
the remaining function lifecycle operations that do not require a metadata
update model:

• createFunction (POST .../functions)
• dropFunction (DELETE .../functions/{function})
• functionExists (HEAD .../functions/{function})

createFunction takes a new CreateFunctionRequest carrying the function name
and its definitions. The server assigns function-uuid, format-version and
definition-log, so those are not part of the request. It returns the loaded
function so clients do not need a follow-up load to learn the assigned UUID
and metadata location.

Errors use IcebergErrorResponse to match the existing function endpoints, and
a FunctionAlreadyExistsError example is added for the 409 case.

updateFunction is left for a follow-up, since committing changes to a function
requires a metadata update and requirement model that the UDF spec does not
define yet. Rename is also deferred: the function endpoints use
CatalogObjectIdentifier while RenameTableRequest uses TableIdentifier, and
which shape rename should use is worth settling separately.

[wombatu-kun]

functionExists declares no 403 while listFunctions, loadFunction, createFunction and dropFunction all do, and it leaves 400/401/404 as bare descriptions while $ref-ing the JSON-bodied 419/503/5XX responses in the same block. Mirror tableExists: $ref BadRequestErrorResponse, UnauthorizedResponse and ForbiddenResponse, and return IcebergErrorResponse with the NoSuchFunctionError example on 404.