Add preference for phone validation

[seand7565]

Description
Adds phone number validation preference so users can easily indicate
whether or not they want to require phone numbers for addresses.
Ref: #3679

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)
• I have attached screenshots to this PR for visual changes (if needed)

[jarednorman]

I think most stores are going to want to keep the phone requirement, aren't they? Pretty much every ecommerce experience I've ever used has, if for nothing else then for support purposes. I don't think we should change the default here, though I'm all for making it configurable if that's something people want to configure.

[seand7565]

I've had the opposite experience - The stores I've worked with prefer not to collect phone numbers (more fields on checkout -> lower conversion, at least in theory) and personally it's something that I'd prefer not to give out as a consumer, especially if there's no obvious reason for it - it just feels like data collection for the sake of data collection.

I'd like Solidus to be less opinionated about what is and isn't required during checkout - IMO anything not immediately necessary to pay for and ship the order should be optional by default.

[jarednorman]

All fair points. I wish we had a way to gather info about what stores needed more generally. I can only speak to the fact that all of the stores we're currently managing (five or so) require phone and will have to change this setting from the new default if we adopt this. It's obviously only an extremely minor inconvenience, but it makes me second-guess whether it's the right call or whether this is going to be a minor inconvenience to quite a lot of stores.

[kennyadsl]

I'd also like to ask less info to users during checkout but I think it's better to keep the old behavior as default without real data to confirm our ideas.