feat(consortium-indy): support consortium database indy backend

[petermetz]

Is your feature request related to a problem? Please describe.

Currently the only consortium plugin we have is the manual one which is very bare bones, basically the consortium plugin equivalent of what the in-memory keychain plugin is for keychains (e.g. it's mostly just good for writing tests without having to simulate costly additional infrastructure software for test cases).

Describe the solution you'd like

The implementation should have the same endpoints/methods as the manual consortium plugin with the big difference being that the consortium database data is being pulled from an Indy ledger rather than from in-memory.
This way the consensus around who is in the consortium is outsourced to the Indy ledger (with all of it's problems which is great for us).
Essentially the plugin is just a resolver for DIDs on Indy.

Describe alternatives you've considered

Nothing, I'm pretty sure we need this.

Additional context

I put this under the epic of pluggable authentication but it's a slightly different topic (lines are blurred)

cc: @takeutak @sfuji822 @hartm @jonathan-m-hamilton @AzaharaC @jordigiam @kikoncuo @mwklein