feat(p/demo/merkle): add constructor w/ getter for merkle.Node struct

[MikaelVallenet]

Hello,

This pull request asks to modify the merkle tree package by adding a constructor for Node and add Position getter
The reason is that in my usage, I receive a json string containing the merkle proofs that I transform into a merkle.Node struct, but since these fields are not exported and no constructor available, I can't do it.

Example of the usage of modified pkg:

func ClaimJSON(airdropID uint64, proofsJSON string) {
	nodes, err := json.Unmarshal([]byte(proofsJSON))
	if err != nil {
		panic("invalid json proofs")
	}
	vals := nodes.MustArray()
	proofs := make([]merkle.Node, 0, len(vals))
	for _, val := range vals {
		obj := val.MustObject()
		data, err := hex.DecodeString(obj["hash"].MustString())
		if err != nil {
			panic("invalid hex encoded hash")
		}
                node := merkle.NewNode(data, jsonutil.MustUint8(obj["pos"])
		proofs = append(proofs, node)
	}
	Claim(airdropID, proofs)
}

P.S: I thought adding getter and constructor would avoid to break contract that already use this like the demo airdrop, it's why i prefer this method over exporting field from the struct

---

Related PRs:

• https://github.com/TERITORI/teritori-dapp/pull/1263/files#diff-2f2cc68375778c01df63bbfb10c4d173f310e653f66e8cb3a4d2c400d1d18aa4

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.43%. Comparing base (f04ec89) to head (a725789).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2784      +/-   ##
==========================================
- Coverage   60.44%   60.43%   -0.01%     
==========================================
  Files         563      563              
  Lines       75159    75159              
==========================================
- Hits        45427    45422       -5     
- Misses      26343    26349       +6     
+ Partials     3389     3388       -1     

Flag	Coverage Δ
contribs/gnodev	61.46% <ø> (ø)
contribs/gnofaucet	14.46% <ø> (-0.86%)	⬇️
gno.land	67.21% <ø> (ø)
gnovm	64.46% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	19.88% <ø> (ø)
tm2	62.05% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.