(10/17) Creates `XRPOfferCancel` Protobuf Wrapper Class

[amiecorso]

High Level Overview of Change

This PR implements the class XRPOfferCancel - a native Swift struct to wrap the OfferCancel protocol buffer. See also https://xrpl.org/offercancel.html

Context of Change

Effort to implement additional XRPL transaction types beyond Payment in the SDK.

Type of Change

• [x] New feature (non-breaking change which adds functionality)
• [x] Tests (You added tests for code that already exists, or your new feature included in this PR)

Before / After

XRPOfferCancel class and conversion unit tests now exist.

Test Plan

Unit tests added to verify conversion. CI passes.

Future Tasks

[codecov[bot]]

Codecov Report

Merging #279 into master will increase coverage by 0.45%. The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
+ Coverage   90.40%   90.86%   +0.45%     
==========================================
  Files          48       55       +7     
  Lines        1105     1215     +110     
==========================================
+ Hits          999     1104     +105     
- Misses        106      111       +5     

Impacted Files	Coverage Δ
...XRPEscrowCancel+Org_Xrpl_Rpc_V1_EscrowCancel.swift	90.00% <90.00%> (ø)
...Model/XRPCheckCash+Org_Xrpl_Rpc_V1_CheckCash.swift	93.33% <93.33%> (ø)
...XRPEscrowFinish+Org_Xrpl_Rpc_V1_EscrowFinish.swift	94.44% <94.44%> (ø)
...XRPEscrowCreate+Org_Xrpl_Rpc_V1_EscrowCreate.swift	95.45% <95.45%> (ø)
...l/XRPCheckCreate+Org_Xrpl_Rpc_V1_CheckCreate.swift	95.83% <95.83%> (ø)
...epositPreauth+Org_Xrpl_Rpc_V1_DepositPreauth.swift	100.00% <100.00%> (ø)
...l/XRPOfferCancel+Org_Xrpl_Rpc_V1_OfferCancel.swift	100.00% <100.00%> (ø)
... and 4 more

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