[Feature request] Add timestamp when transaction has been boardcasted/seen first.

[SeverinAlexB]

Describe the feature
I am creating a tool that does batch payouts at the lowest fee possible. After 24hrs, if not confirmed yet, I want to execute a fee bump.
It would be very helpful to have a timestamp when the transactions has been boardcasted/first seen. At the moment, I need to create an additional database for that.

Best regards
Severin

[LLFourn]

I suppose we always "see" a transaction at a certain point and can record it so I think this is doable. I'll keep this in mind for bdk_core.

[remix7531]

I will work on this issue.

[evanlinjin]

I will work on this issue.

This is already in-progress with the work on the bdk core redesigned structures. #926 #943

[notmandatory]

This issue should be fixed in #976, that PR includes the field CanonicalTx.observed_as of type ChainPosition which has a variant Unconfirmed(u64) which encapsulates when it was seen in the mempool.

[notmandatory]

@SeverinAlexB please checkout current master branch now that #976 is merged and let us know if this gives you what you need.