Add publicClient property in hardhat-viem

[kanej]

The Hardhat 3 version of hardhat-viem should have a publicClient property in addition to the getPublicClient getter. We didn't do that in v2 because viem clients are built asynchronously, and the HRE needed to be built synchronously. But in v3, the viem object is returned by the async network.connect method, so we can create and add a client.

The reason this is valuable is that it makes less verbose the very verbose viem usage:

// before
const { viem } = await network.connect()
const publicClient = await viem.getPublicClient();
console.log(await publicClient.getBlockNumber())

// after
const { viem } = await network.connect()
console.log(await viem.publicClient.getBlockNumber())

There's probably other things we could look into here. For example, maybe we could have a viem.client property, which only has public actions if there are no configured wallets, has test actions when connecting to a Hardhat network, etc. But I think we should keep it simple. The publicClient property is obvious and unlikely to be a bad call, and we can add other things later in a backwards compatible way.