add api for retrieving getblockchaininfo RPC result

Many properties of the chain, especially for liquid, are reported by the `getblockchaininfo` RPC, that are not easily or directly retrievable from the current api.

An example of a property is the current `fedpegscript`, which is reported in the first block in each dynafed epoch (20160 for liquid mainnet). However the 20160 epoch length is a chain parameter, also reported by `getblockchaininfo`.

Adding a API resource that returns the current `getblockchaininfo` would assist thin clients in determining properties such as `fedpegscript`.

For bitcoin mainnet this resource would also be useful.