[CAL][7] - implement GetExpectedNextSequenceNumber

[ogtownsend]

core ref: aad88a584c81e0be68f68927d5ab041fc65ca646

• Implement GetExpectedNextSequenceNumber()
• 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

[github-actions]

Metric	ogt/cal-7-implement-GetExpectedNextSequenceNumber	ogt/cal-6-migrate-SourceChainConfig-to-types-package
Coverage	71.3%	70.9%