[TierTwo] P2SH Proposal Payouts

[panleone]

the idea of this PR is taken from #2265.
Having the governance system accept P2SH proposals is a nice addition since, for example, the work of a team can be paid in a single multisig address.
P2SH proposals will be enabled after v6.0 and is not backward compatible: in particular with the current implementation a v6 node will relay a p2sh proposal even to non-v6 nodes (and will lead to non v6 nodes banning the v6 node).
Since v6.0 will be a huge mandatory update I don't think it will be an issue

[panleone]

Ready for review: last commit is a bit out of scope and adds CheckString() in IsWellFormed() (It was a waste of time opening another PR just for that line)

[RainDeveloper69]

Thanks for working on enabling P2SH/multisig proposal payouts.

I reviewed the current implementation and found one issue that I think should be addressed before approval.

CheckStrings() can reject valid proposal URLs

After v6.0 activation, IsWellFormed() now calls CheckStrings():

if (isV6Enforced && !CheckStrings()) {
    return false;
}

CheckStrings() uses SanitizeString() with SAFE_CHARS_DEFAULT, which does not allow several standard URL characters such as &, =, #, %, and +.

This means valid proposal URLs containing query parameters or fragments may be rejected after v6.0 activation.

For example:

https://example.com/page?foo=1&bar=2

or:

https://example.com/proposal#details

would fail the current validation.

There is already a dedicated validateURL() helper in utilstrencodings, so it may be worth considering URL-specific validation here instead of using the generic SanitizeString() check.

The added functional test covering the P2SH proposal flow is useful, but I would also recommend adding regression coverage for valid URLs containing query parameters and fragments.

I don't see a consensus-level issue with the core P2SH payout change itself, but I think the URL validation behavior should be clarified or fixed before this PR is approved.