(16/17) Creates `XRPSignerListSet` and `XRPSignerEntry` Protobuf Wrapper Classes

[amiecorso]

High Level Overview of Change

This PR implements the class XRPSignerListSet and a dependency XRPSignerEntry - native Swift structs to wrap the XRPSignerListSet protocol buffer (See also https://xrpl.org/signerlistset.html), and the XRPSignerEntry protocol buffer (see also https://xrpl.org/signerlist.html#signerentry-object)

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

XRPSignerListSet class and conversion unit tests now exist. XRPSignerEntry class and conversion unit tests now exist.

Test Plan

Unit tests added to verify conversion. CI passes.

Future Tasks

[codecov[bot]]

Codecov Report

Merging #285 into master will increase coverage by 0.51%. The diff coverage is 93.30%.

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
+ Coverage   90.28%   90.79%   +0.51%     
==========================================
  Files          46       62      +16     
  Lines        1091     1315     +224     
==========================================
+ Hits          985     1194     +209     
- Misses        106      121      +15     

Impacted Files	Coverage Δ
...XRP/Model/XRPPayment+Org_Xrpl_Rpc_V1_Payment.swift	100.00% <ø> (ø)
...nelClaim+Org_Xrpl_Rpc_V1_PaymentChannelClaim.swift	86.66% <86.66%> (ø)
...annelFund+Org_Xrpl_Rpc_V1_PaymentChannelFund.swift	86.66% <86.66%> (ø)
...lCreate+Org_Xrpl_Rpc_V1_PaymentChannelCreate.swift	87.50% <87.50%> (ø)
...XRPEscrowCancel+Org_Xrpl_Rpc_V1_EscrowCancel.swift	90.00% <90.00%> (ø)
...l/XRPOfferCreate+Org_Xrpl_Rpc_V1_OfferCreate.swift	92.30% <92.30%> (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)
... and 23 more

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