feat(esplora): make ext traits more flexible

feat(esplora): make ext traits more flexible

Some users would like to use esplora updates with custom `ChainOracle`s
(not `LocalChain`). We introduce "low-level" extension methods that
populate an update `TxGraph` with associated data.

Additionally, much of the logic has been made more efficient. We make
use of the `/tx/:txid` endpoint (`get_tx_info` method) to do a single
call to get both the tx and tx_status. If the tx already exists, we only
fetch the tx_status. The logic for fetching data based on outpoints has
been reworked to be more efficient and it now supports parallelism
Additionally, the logic for fetching data based on outpoints has been
reworked to be more efficient and it now supports parallelism.

Documentation has also been reworked.

Note that this is NOT a breaking change because the API of `full_scan`
and `sync` are not changed.