Query Cantillon worker's TrustedGetter from js_servcie_encointer (Balance, registrations, meetup info)

[brenzi]

For the app to work with Cantillon, we'll need to implement the worker-api for the following Getters:

Assumptions:

• we use the same on-chain account also as our incognito account for offchain workers for simplicity (rust client can use different keypairs for on-chain and off-chain state)

Rust definitions for TrustedGetter:
https://github.com/encointer/encointer-worker/blob/35ae50a27b293971f2289d71ef3f81b604ae8d33/stf/src/lib.rs#L112-L117

rust client commands:

1. encointer-client-teeproxy trusted balance .... Rust
2. encointer-client-teeproxy trusted get_registration .... Rust
3. encointer-client-teeproxy trusted get_attestations .... Rust
4. get meetup assignment for claim generation: Rust

The TrustedGetter has to be wrapped multiple times:

1. into a TrustedGetterSigned (by signing it with our incognito account = polkawallet account)
2. into a ClientRequest Definition

then send request over websocket to : wss://substratee03.scs.ch
return value will be a hex::encode(SCALE::encode(value)). Can only be decoded if type is known:

1. Balance is a BalanceType
2. registration is ParticipantIndexType
3. attestations is Vec<Attestation>
4. meetup assignment is (MeetupIndexType, Option<Location>, Option<Moment>)

We may want to query public stats too once they are available. See:
worker #7

[brenzi]

If possible, we should keep this +- compatible with the explorer in order to solve this one easily afterwards:
encointer/explorer#12

[brenzi]

since worker v0.6.7 TrustedGetterSigned needs to be wrapped into a Getter before wrapping it into ClientRequest

[clangenb]

Basic functionality has been implemented. We are able to talk to the worker.

However, some return values are weird. Track remaining issues in #30