internal/ethapi: rename debug getters to match spec

[lightclient]

(waiting for spec to be merged: https://github.com/ethereum/execution-apis/pull/247)

Some small fixes to get the existing debug methods to conform to the spec. Mainly dropping the encoding information from the method name as it should be deduced from the debug context and allowing the method to be invoked by either block number or block hash. It also adds the method debug_getTransaction which returns the raw tx bytes by tx hash. This is pretty much equivalent to the eth_getRawTransactionByHash method.

[fjl]

Can you un-draft this, please?