Initial PSS ADR and notes

[jtremback]

Description

Closes: #XXXX

Related to #853

This is a start to an ADR for Partial Set Security, as well as a set of notes in the code about places that may need to be modified to implement it. Many questions remain on the design so the ADR currently summarizes some of them and answers others.

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Author Checklist

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• included the correct docs: prefix in the PR title
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[MSalopek]

Closing as it is superseded by https://github.com/cosmos/interchain-security/blob/main/docs/docs/adrs/adr-015-partial-set-security.md