feat(chain): Txs that conflict with relevant txs are also relevant

feat(chain): Txs that conflict with relevant txs are also relevant

Change behavior of {insert|apply}-if-relevant methods of `IndexedTxGraph`
to also consider txs that conflict with relevant txs as relevant.

Rationale:

It is useful to determine why something is evicted from the mempool.

For example, an incoming transaction may be evicted from the mempool due
to insufficient fees or a conflicting transaction is confirmed.

* Insufficient fees - the user may want to CPFP the tx.
* Conflicting tx is confirmed - the sender probably purposefully
  cancelled the tx. The user may want to forget about this tx once it
  reaches x confirmations.

The `IntentTracker` will make use of these relevant-conflicts.

A note about chain sources:

For some chain sources, obtaining relevant-conflicts is extremely
costly or downright impossible (i.e. Electrum, BIP-158 filters).

`bdk_bitcoind_rpc::Emitter` is still the most robust chain source to use.