archive/storage: Use a similar API with chainHead

[lexnv]

The following changes are done to the archive methods:

• Use integers for describing block heights instead of strings containing integers
• Provide a similar API for archive_storage as for the chainHead_storage
• Use camel case for event description

cc @tomaka @jsdw @josepot @paritytech/subxt-team

[tomaka]

This should IMO be multiple different PRs. I don't see why they're bundled in one.

[lexnv]

That's a good point! Thanks!

Closing this in favor of:

• archive: Use integer for describing block height #92
• archive: Use camelCase for waitingForContinue event #93
• archive/storage: Backport API from chainHead #94
• archive/body: Keep description consistent with chainHead #95