Add a default validation policy

[hannahhoward]

Goals

In untrusted context, limit recursive selectors to a fixed depth to prevent some DOS attacks. (should have been done a while ago)

Implementation

• When a request is received, examine the passed selector by (perhaps a bit meta) traversing the selector spec with a selector, checking all recursive selectors have a depth <= 100 (current fixed depth limit)
• Does not include customizing allowed max depth for now (future ticket)
• Does not rule out other kinds of selectors yet
• also simplifies request tracking in response manager by saving whole request instead of individual components
• also removes the BuildSelectorSpec method from IPLD bridge because it's a very thin wrapper on ipld prime that needs no mocking.

[Stebalien]

I think we'll have to revisit this later to take fan-out into account. 100 is small for a chain, but possibly large for a large fanout.

However, we can handle this later. This validation is strictly better than what we have and is a clean way to throw out known-bad selectors.

[anorth]

FWIW I think this default is more trouble than it will be worth, and we should do the proper solution now of forcing the constructor/caller to specify a value here. It shows immediately in a downstream PR where there's a new flag for "override default validation" explicitly to work around this.