rpc-v2: Enable the `archive` class of methods (#3017)

The
[archive](https://github.com/paritytech/json-rpc-interface-spec/blob/main/src/api/archive.md)
API is unstable and subject to change.

This PR enables the `archive` class of the RPC-V2 spec to substrate
based chains.

The `archive` API is enabled for archive nodes: 
- the state of the blocks is in archive mode
- the block's bodies are in archive mode

While at it, this PR extends the `BlocksPrunning` enum with an
`is_archive` helper to check if the pruning mode keeps the block's
bodies for long enough.

Defaults used for the `archive` API:
- a maximum of 5 responses are provided for descendants queries (this is
similar to chainHead)
- a maximum of 8 item queries are accepted at a time

Before stabilizing the API we should look into these defaults and adjust
after collecting some data.

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>