[CAL][5] - implement LatestMsgSeqNum

[ogtownsend]

core ref: aad88a584c81e0be68f68927d5ab041fc65ca646

• Implement MsgsBetweenSeqNums() functions for Chain Access Layer in legacy accessor
• For more context, see NONEVM-1865

Stack:

1. [CAL][1] - Add initial CAL scaffolding to CCIPReader with legacy accessor #978
2. [CAL][2] - Implement Metadata() and GetContractAddress() #980
3. [CAL][3] - Implement GetChainFeeComponents #985
4. [CAL][4] - Implement MsgsBetweenSeqNums() function in accessor #987
5. ➡️ [CAL][5] - implement LatestMsgSeqNum #988
6. [CAL][6] - move SourceChainConfig to chainaccessor types #989
7. [CAL][7] - implement GetExpectedNextSequenceNumber #990
8. [CAL][8] - Some minor cleanup and renaming #999
9. [CAL][9] - Implement CommitReportsGTETimestamp, GetTokenPriceUSD, and GetLatestPriceSeqNr #1000
10. [CAL][10] - Implement Nonces and GetChainFeePriceUpdate #1014
11. [CAL][11] - Implement ExecutedMessages for chain accessor #1017

The base branch was changed.

[github-actions]

Metric	ogt/cal-5-implement-latest-msg-seq-num	main
Coverage	71.3%	71.2%