fix: make MockSignature signature field public #3799

Job	Run time
Cancel previous runs	7s
Generate test coverage	7m 51s
Build Clarity REPL Distribution	3m 44s
Get Release Info	5s
Audit and format	14s
Build Clarinet Distributions (ubuntu-latest, linux, x86_64-unknown-linux-gnu, x64, glibc)	11m 9s
Build Clarinet Distributions (windows-latest, windows, x86_64-pc-windows-msvc, x64)	30m 37s
Build Clarinet Distributions (macos-latest, darwin, x86_64-apple-darwin, x64)	8m 41s
Build Clarinet Distributions (macos-latest, darwin, aarch64-apple-darwin, arm64)	8m 12s
Build Stacks Devnet JS Distributions (ubuntu-latest, linux, x86_64-unknown-linux-gnu, x64, glibc)	8m 5s
Build Stacks Devnet JS Distributions (windows-latest, windows, x86_64-pc-windows-msvc, x64)	16m 40s
Build Stacks Devnet JS Distributions (macos-latest, darwin, x86_64-apple-darwin, x64)	6m 28s
Build Stacks Devnet JS Distributions (macos-latest, darwin, aarch64-apple-darwin, arm64)	6m 31s
Build Stacks Devnet JS Distributions (ubuntu-latest, linux, x86_64-unknown-linux-musl, x64, musl)	16m 5s
Create Clarinet Docker Image	10m 41s
Release	0s
	2h 15m 10s

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