Expose consensus params from algod

Use case:
SDKs need to know some consensus params (examples are max TEAL cost or max ASA URL length.

Suggested implementation:
1. Add REST API endpoint that returns params by protocol name as JSON (or all supported protocols and params)
2. JSON fields must match `ConsensusParams` struct from **config/consensus.go**
3. Add `go:generate` command that dumps all consensus version and params as a single JSON file similarly to TEAL's **langspec.json**

SDKs receive the file from p.3. SDKs are also capable to use actual params by querying the endpoint from p.1

This is similar to TEAL approach: SDKs can work offline using TEAL spec or query the compile endpoint.