Merge bitcoindevkit/bdk#779: Add signature grinding for ECDSA signatures

68dd6d20311b28f5c8e8c9657dce8cddae4f7aa3 Add signature grinding for ECDSA signatures (Vladimir Fomene)

Pull request description:

  ### 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.

  ### Changelog notice

  Breaking change: the BDK Signer now produces low-R signatures by default, saving one byte. If you want to preserve the original behavior, set `allow_grinding` in the `SignOptions` to `false`.

  ### Notes to the reviewers

  This PR resolves issue #695

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [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

ACKs for top commit:
  danielabrozzoni:
    ACK 68dd6d20311b28f5c8e8c9657dce8cddae4f7aa3
  rajarshimaitra:
    ACK 68dd6d20311b28f5c8e8c9657dce8cddae4f7aa3

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