Use BigInt in CT Module

[mj850]

Describe your changes and provide context

We currently represent balances in the CT module as uint64s. However, it is possible that the balances exceed the limits of uint64s, especially for decrypted available balances and pending balances.

We need to use big.Ints instead of uint64 to represent balance values.

Testing performed to validate your change

Updated Unit tests and integration tests

[codecov]

Codecov Report

Attention: Patch coverage is 26.31579% with 56 lines in your changes missing coverage. Please review.

Project coverage is 59.96%. Comparing base (81c0d5f) to head (b0c9303).
Report is 44 commits behind head on feature/ct_types.

Files with missing lines	Patch %	Lines
x/confidentialtransfers/types/transfer.go	0.00%	25 Missing ⚠️
x/confidentialtransfers/types/account.go	0.00%	8 Missing ⚠️
.../confidentialtransfers/types/initialize_account.go	0.00%	8 Missing ⚠️
x/confidentialtransfers/types/withdraw.go	0.00%	8 Missing ⚠️
x/confidentialtransfers/types/msgs.go	63.63%	3 Missing and 1 partial ⚠️
...nfidentialtransfers/types/apply_pending_balance.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##           feature/ct_types    #1966      +/-   ##
====================================================
- Coverage             61.57%   59.96%   -1.62%     
====================================================
  Files                   263      283      +20     
  Lines                 23306    25883    +2577     
====================================================
+ Hits                  14351    15520    +1169     
- Misses                 7948     9224    +1276     
- Partials               1007     1139     +132     

Files with missing lines	Coverage Δ
x/confidentialtransfers/keeper/msg_server.go	73.68% <100.00%> (ø)
x/confidentialtransfers/utils/utils.go	100.00% <100.00%> (ø)
...nfidentialtransfers/types/apply_pending_balance.go	0.00% <0.00%> (ø)
x/confidentialtransfers/types/msgs.go	39.91% <63.63%> (ø)
x/confidentialtransfers/types/account.go	0.00% <0.00%> (ø)
.../confidentialtransfers/types/initialize_account.go	0.00% <0.00%> (ø)
x/confidentialtransfers/types/withdraw.go	0.00% <0.00%> (ø)
x/confidentialtransfers/types/transfer.go	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes