Hoist blockTag into client

[shazow]

There's already experimental_blockTag which supports string block tags, but would be great if we could make this officially supported (non-experimental) and allow numeric blocks too.

Rationale: It's more common/ergonomic to always query within the context of a specific block. It's annoying to pass that same block in everywhere, so fixing that block as a client configuration is a nice abstraction.

Specifically, @andrewhong5297 was using whatsabi to analyze a recent hack against a specific block, and it was quite annoying to instrument whatsabi to plumb the blockNumber. I added a WithBlockNumber(provider, blockTag) helper which wraps the provider (essentially faking the hoisting), will be included in the next release (shazow/whatsabi#198).

The other option was to use something like tevm to provide a forked client against some block number and pass that as a provider instead.

In either case, it feels like it makes sense to be able to hoist/fix the block number within the context of a client/provider. :)