Need for KPIs for the new website

### Is your feature request related to a problem?

We need some KPI for the new website (prequisites).

Ideally, it would be interesting to have data in big data but there is a reflection to make on the decentralization of such a solution.
The demand being short term, is it possible to have APIs to obtain the following information (not necessarily real time...)

- Nb of transactions globally (same as KPI on https://mainnet.archethic.net/explorer)
- Nb of nodes available and authorized (i can use GraphQL + filters for the moment)
- Transaction Validation Time
- Nb of keychains globally

### Describe the solution you'd like

_No response_

### Additional context

_No response_

No file, test, or entry point is named. Start by reviewing the existing GraphQL API and the mainnet explorer KPI for transaction counts, then determine where node availability, validation time, and global keychain counts could be exposed. Done requires an agreed API design for all four metrics, including freshness and decentralization considerations.

Written by the indexing model from the issue text.