Add signature grinding for ECDSA signatures

[vladimirfomene]

Description

This PR adds a new field called allow_grinding in the Signer's SignOptions struct that is used to determine whether or not to grind an ECDSA signature during the signing process.

Notes to the reviewers

This PR resolves issue #695

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing

New Features:

• [ ] I've added tests for the new feature
• [x] I've added docs for the new feature

Bugfixes:

• [ ] This pull request breaks the existing API
• [ ] I've added tests to reproduce the issue which are now passing
• [x] I'm linking the issue being fixed by this PR