[FEAT] Length task to return the length of a JSON Array

Description

Currently the Length Task only returns the length of strings or byte arrays. It would be very valuable if the task also supported JSON Arrays.

length_myvar      [type="length" input="$(myvar)"] // Being 'myvar' a JSON Array, e.g. [1, 2, 3]

Motivation

I have a TOML spec that requires first to read from a mapping(uint256 => uint256[]) (via ethcall) and ideally make the job fail fast if the uint256[] is empty.

Testing Consumer on ETH Goerli:
https://goerli.etherscan.io/address/0xB056566a48F29cDEFbAF60Ac31C75370e5a39623#readContract#F1

Testing TOML spec:

NB: _sportId: 1 will output [] at the end of the job run, whilst _sportId: 18 will output [3, 11].

type = "webhook"
schemaVersion = 1
name = "Test Get Bookmaker IDs"
observationSource = """
    encode_bookmakerids_var_call    [type="ethabiencode" abi="getBookmakerIdsBySportId(uint256 _sportId)" data=<{ "_sportId": 1}>]

    call_bookmakerids_var           [
                                     type="ethcall"
                                     contract="0xB056566a48F29cDEFbAF60Ac31C75370e5a39623"
                                     data="$(encode_bookmakerids_var_call)"
                                     extractRevertReason=true
                                    ]
    decode_bookmakerids             [type="ethabidecode" abi="uint256[] bookmakerIds" data="$(call_bookmakerids_var)"]

encode_bookmakerids_var_call -> call_bookmakerids_var -> decode_bookmakerids
"""

For sportId = 1 the decode_bookmakerids output is []. If length accepted JSON Arrays I could use the lessthan and conditional tasks to make it fail fast, e.g.

type = "webhook"
schemaVersion = 1
name = "Test Get Bookmaker IDs - If length supported JSON Arrays"
observationSource = """
    encode_bookmakerids_var_call    [type="ethabiencode" abi="getBookmakerIdsBySportId(uint256 _sportId)" data=<{ "_sportId": 1}>]

    call_bookmakerids_var           [
                                     type="ethcall"
                                     contract="0xB056566a48F29cDEFbAF60Ac31C75370e5a39623"
                                     data="$(encode_bookmakerids_var_call)"
                                     extractRevertReason=true
                                    ]

    decode_bookmakerids             [type="ethabidecode" abi="uint256[] bookmakerIds" data="$(call_bookmakerids_var)"]

    length_bookmakerids             [type="length" input=$(decode_bookmakerids.bookmakerIds)]

    is_bookmakerids_empty           [type="lessthan"  left="$(length_bookmakerids)" right=1]

    // NB: revert before going further executing tasks, e.g. requesting the bridge
    revert_if_empty                 [type="conditional" data="$(is_bookmakerids_empty)"]

encode_bookmakerids_var_call -> call_bookmakerids_var -> decode_bookmakerids -> length_bookmakerids -> is_bookmakerids_empty -> revert_if_empty
"""

Justification

Increase the offchain computation capabilities without having recurr to any of the following strategies:

• EA-based: either there is an endpoint that calculates the length or the length of N JSON result properties are part of the EA result.
• Contract-based: there is a callable method (either pure or view) that gets/calculates lengths on-demand.

Mroeover, I've been recently using mode and length could be handy to calculate the length of the results.

On a similar topic I'd like to request support JSON Arrays as $(my_array_var) on any built-in task with the parameter values, e.g. mean, median, mode, sum, etc. It would then be possible to use them with on-chain arrays (of unknown size/items) via ethcall.

Thanks as always

Start at the Length Task implementation and its existing handling for strings and byte arrays. Add support for JSON Arrays, including empty and non-empty arrays, and verify that the provided examples produce the expected lengths so downstream lessthan and conditional tasks can use them.