Refactor how DIDs and Verida networks work

[tahpot]

Key requirements:

1. A DID document needs to become completely independent from the Verida network
2. A single DID document can be linked to contexts across different Verida networks
3. Non-Verida DIDs can be used on the Verida network

We will support a new DID method format for VDA DIDs: did:vda:[anchored-blockchain]:0x…. Here are examples:

1. did:vda:amoy:0x..
2. did:cheqd:0x..
3. did:base:0x..
4. did:polygonid:polygon:main:0x
5. did:polygonid:polygon:mumbai:0x

Anchored blockchains will be a human readable name, that is effectively a lookup within the Verida DID resolver:

1. polamoy: Polygon Amoy
2. polpos: Polygon PoS
3. polzkevm: Polygon zkEVM
4. base: Coinbase Mainnet
5. basesepolia: Coinbase Sepolia

We need to add &network= to DID Document / serviceEndpoint / id which specifies the Verida network associate with that context.

The Verida SDK will support three Verida networks:

1. devnet
2. myrtle
3. banksia
4. mainnet (for backwards compatibility, is actually myrtle)