Block-Version-based Protocol Evolution (#26)

* A proposal to use `block_version` to control schema evolution

* add future possibilities mention

* update for review comments

* add a further sentence about motivation (from comment discussion)

* add MCIP links

* add more MCIP links

* rewrote rationale and alternatives

to more properly describe the alternative roll out for encrypted memos

* fix mcip references

* Update text/0026-block-version-based-protocol-evolution.md

Co-authored-by: Remoun Metyas <[email protected]>

Co-authored-by: Remoun Metyas <[email protected]>