MINOR: http_fetch: Add support for checks to `unique-id` fetch

MINOR: http_fetch: Add support for checks to `unique-id` fetch

This allows to use the `unique-id` fetch within `tcp-check` or `http-check`
ruleset. The format is taken from the checked server's backend (which is
naturally inherited from the corresponding `defaults` section).

This is particularly useful with

    http-check send ... hdr request-id %[unique-id]

to ensure all requests sent by HAProxy have a unique ID header attached.

This resolves GitHub Issue #3307.

Reviewed-by: Volker Dusch <github@wallbash.com>