ecies: add version byte prefix to encoding format

[ffranr]

Extend the encoding format to include a version byte prefix. This helps facilitate cleaner and safer upgrades in the future.

[guggero]

Very nice, thank you! LGTM 🎉

The litd itest is going to be fixed by #1614, so we can ignore that here.

[coveralls]

Pull Request Test Coverage Report for Build 15873552727

Details

• 33 of 40 (82.5%) changed or added relevant lines in 1 file are covered.
• 20 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.08%) to 38.672%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/ecies/ecies.go	33	40	82.5%

Files with Coverage Reduction	New Missed Lines	%
internal/ecies/ecies.go	2	78.69%
tappsbt/create.go	2	26.74%
tapdb/multiverse.go	6	53.28%
tapgarden/caretaker.go	10	67.63%

Totals
Change from base Build 15865241809:	0.08%
Covered Lines:	29639
Relevant Lines:	76643

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💛 - Coveralls