Include `electrsd` in the workspace

[tcharding]

Currently electrsd is excluded from the workspace. One reason is that cargo test --all-features does not work for electrsd.

Likely we can do the same feature gate dance that we do in bitcoind to get this working.

To close this issue:

• Do what ever is needed to get --all-features working
• Configure cargo rbmt stuff as needed in electrsd manifest.
• Move electrsd to the members array in the repo manifest.

[tcharding]

cc @satsfy since cargo rbmt is fresh in your mind. Just if you feel like more CI work mate.