Allow a conn open ack to succeed in the happy case

[vitorenesduarte]

Closes: #699

Description

Currently, the conn open ack handler requires a connection to have the counterparty connection id set (line 36):

https://github.com/informalsystems/ibc-rs/blob/02aef9cf5055a3817579b555c84289b99d5d08a8/modules/src/ics03_connection/handler/conn_open_ack.rs#L33-L45

However, if the connection is in the Init state (the happy/normal case), the counterparty connection id is unknown (i.e. a None), in which case the conn open ack fails with a ConnectionMismatch.

With this PR, we allow the counterparty connection id to be None, and only fail with a ConnectionMismatch in case it's a Some (that doesn't match).

Since theMsgConnectionOpenAck.counterparty_connection_id field is always required, this PR also changes it to simply be a ConnectionId (instead of an Option<ConnectionId>).

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For contributor use:

• Updated the Unreleased section of CHANGELOG.md with the issue.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[vitorenesduarte]

This test has been removed as my understanding is that msg.counterparty_connection_id should always be a Some (which hints that it should not be an Option).

[vitorenesduarte]

Now this field is no longer an Option.

[vitorenesduarte]

If the connection is in the Init state, connection_end.counterparty().connection_id() is a None, and thus it's not okay to unwrap.

[codecov-io]

Codecov Report

Merging #700 (731dcc9) into master (b1b37f5) will increase coverage by 30.4%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #700      +/-   ##
=========================================
+ Coverage    13.6%   44.1%   +30.4%     
=========================================
  Files          69     150      +81     
  Lines        3752    9764    +6012     
  Branches     1374       0    -1374     
=========================================
+ Hits          513    4312    +3799     
- Misses       2618    5452    +2834     
+ Partials      621       0     -621     

Impacted Files	Coverage Δ
modules/src/address.rs	100.0% <ø> (ø)
...application/ics20_fungible_token_transfer/error.rs	0.0% <ø> (ø)
...ion/ics20_fungible_token_transfer/msgs/transfer.rs	0.0% <ø> (ø)
modules/src/events.rs	0.0% <ø> (ø)
modules/src/handler.rs	100.0% <ø> (ø)
modules/src/ics02_client/client_def.rs	48.3% <ø> (ø)
modules/src/ics02_client/client_type.rs	79.1% <ø> (+31.5%)	⬆️
modules/src/ics02_client/context.rs	100.0% <ø> (ø)
modules/src/ics02_client/error.rs	100.0% <ø> (ø)
modules/src/ics02_client/events.rs	15.7% <ø> (+15.7%)	⬆️
... and 255 more

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[adizere]

LGTM, thank you Vitor.