Add ECDSA verification benchmarks

[briansmith]

We need benchmarks that verify ECDSA signatures in order to make progress on various improvements to ECDSA and to support the project of moving from C to Rust.

These benchmarks should use a fixed ECDSA public key loaded from a checked-in file and a fixed (set of) signature(s). Don't generate a new ECDSA keypair as part of the benchmark scaffolding, and don't generate new signatures in the scaffolding, because the ECDSA verification code is variable-time with respect to the bit pattern in the signature (the values of r and s), and we want the benchmarks to provide reproducible/comparable timing.

[briansmith]

PR #1773 provides a good example of how to do this.