CT Transfer types

[dssei]

Describe your changes and provide context

This PR contains transfer types to support confidential transfers feature

Testing performed to validate your change

• Unit testing

[codecov]

Codecov Report

Attention: Patch coverage is 62.76596% with 175 lines in your changes missing coverage. Please review.

Project coverage is 61.37%. Comparing base (dd940e5) to head (a9e6ad5).

Files with missing lines	Patch %	Lines
x/confidentialtransfers/types/zk.go	61.17%	88 Missing and 44 partials ⚠️
x/confidentialtransfers/types/msgs.go	63.04%	25 Missing and 9 partials ⚠️
x/confidentialtransfers/types/codec.go	53.84%	6 Missing ⚠️
x/confidentialtransfers/types/cryptography.go	88.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1899      +/-   ##
==========================================
+ Coverage   61.35%   61.37%   +0.01%     
==========================================
  Files         263      267       +4     
  Lines       23328    23798     +470     
==========================================
+ Hits        14314    14606     +292     
- Misses       8009     8133     +124     
- Partials     1005     1059      +54     

Files with missing lines	Coverage Δ
x/confidentialtransfers/types/cryptography.go	88.00% <88.00%> (ø)
x/confidentialtransfers/types/codec.go	53.84% <53.84%> (ø)
x/confidentialtransfers/types/msgs.go	63.04% <63.04%> (ø)
x/confidentialtransfers/types/zk.go	61.17% <61.17%> (ø)

... and 2 files with indirect coverage changes

[mj850]

This LGTM so far, to summarize my understanding here:

1. client creates 'usable' transfer object usableTransferObject *Transfer -> This is a version of the object where the ciphertexts/curves.Points are in their usable form.
2. client creates the MsgTransfer object by converting usableTransferObject (maybe by calling some function like MsgTransfer.ToProto(usableTransferObject)) - we need to do this conversion because the *Transfer object is not proto marshallable
3. This message is marshalled as a proto and sent to the server side
4. Server side unmarshals the message and it arrives at the handler as a MsgTransfer object.
5. Server converts the MsgTransfer back to a usable transfer object *Transfer by calling MsgTransfer.FromProto()

Does this sound right?

Questions:

1. We need to perform the intermediate steps 2 and 4 because the regular Transfer object (or more specifically the curves.Points and curves.Scalar objects) are not proto marshallable?
2. Since we forked the kryptology library, are there any changes we can make there to enable us to marshal the *Transfer object directly without conversion to MsgTransfer?
3. Should we have some method 'ToProto' that converts a Transfer object to a MsgTransfer?
4. We would likely have to expose the types somehow since we would want external users other than the seid cli to be able to create valid MsgTransfer objects as well. We could add these type in here for now, but it might make sense to think about whether we can eventually make these types and msg generating functions external

[mj850]

testDenom*

[mj850]

Would it make sense to make this its own function?

func ToProto(transfer *Transfer) *MsgTransfer {}