Enable IBD via URL

[BLuEScioN]

Would be neat if we coul set IBD inthe URL of a stable API deployment, and the init code could do "if (idbIsUrl) idbHeight = fetch(idbUrl).blockHeight" ... but that has some complexity and weird implications. Probably best to wait until the stacks-node itself is able to start reporting this value.

[sabbyanandan]

This is not a priority for us, for now.