Add option to update and freeze updating the metadata uri

[314159265359879]

This option provides some more flexibility compared to hard coding the uri yet still makes it immutable after you've checked everything is in order.

Just like in the SP3D6PV2ACBPEKYJTCMH7HEN02KP87QSP8KTEH335.megapont-ape-club-nft (see from line 142 and 150 (and line 019, 037))

Add error
(define-constant ERR-METADATA-FROZEN (err u505))

Add variable
(define-data-var metadata-frozen bool false)
assumes:
(define-data-var base-uri (string-ascii 80) "ipfs://Qmad43sssgNbG9TpC6NfeiTi9X6f9vPYuzgW2S19BEi49m/{id}")

add set base uri
;; Set base uri (define-public (set-base-uri (new-base-uri (string-ascii 80))) (begin (asserts! (is-eq tx-sender CONTRACT-OWNER) ERR-NOT-AUTHORIZED) (asserts! (not (var-get metadata-frozen)) ERR-METADATA-FROZEN) (var-set base-uri new-base-uri) (ok true)))

add Freeze metadata
;; Freeze metadata (define-public (freeze-metadata) (begin (asserts! (is-eq tx-sender CONTRACT-OWNER) ERR-NOT-AUTHORIZED) (var-set metadata-frozen true) (ok true)))