feat: add signature proof spec

signature-proof-spec/outliner.md

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+merkle tree signature proofs outliner:
+
+introduction:
+- introduce the current 3 flavours of VCs reference kaliya young, “identity woman” paper
+- propose an alternative to the 3 flavours (similar to Lightweight selective disclosure for verifiable documents on blockchain)
+- rest of the paper will describe how it works
+design of the proof:
+- overview of credential
+    - short example of a credential (simple example)
+- explain how the data integrity works
+    - talk about the salts (rainbow attacks)
+    - normalising credential into individual key-pairs
+    - hashing each of individual key-pairs
+    - compute targetHash of credential
+    - assembling of credential after calculating proof.
+- selective disclosure
+- merkle hash trees
+    - short introduction
+    - and how it affects current solution so far
+limitations of the proof:
+- data obfuscation limits
+- salt size only grow linearly as more claims are added
+    - might impact performance
+related work:
+- combining subtrees from different claims are not allowed unlike(reference related work Minimal Information Disclosure with Efficiently Verifiable Credentials)
+- the set of claims itself is not a merkle tree unlike [reference A Linearly Homomorphic Signature Scheme From Weaker Assumptions]