feat(b-dropdown): add toggle-attrs prop (closes #3694)

[phegman]

Describe the PR

Add toggle-attrs prop to allow adding HTML attributes to the dropdown toggle. This feature has been talked about here: #3694

At GitLab we have a component library built on top of Bootstrap Vue and would like this feature so that we can add the data-qa-selector attribute to the dropdown toggle. We use this attribute exclusively for QA E2E specs. I could also see it being helpful for adding the data-testid attribute which is supported by Vue Testing Library. This prop could also be used for other user-facing attributes such as aria-* attributes.

Closes #3694.

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix (fixes a boo-boo in the code) - fix(...), requires a patch version update
• Feature (adds a new feature to BootstrapVue) - feat(...), requires a minor version update
• Enhancement (augments an existing feature) - feat(...), requires a minor version update
• ARIA accessibility (fixes or improves ARIA accessibility) - fix(...), requires a patch or minor version update
• Documentation update (improves documentation or typo fixes) - chore(docs), requires a patch version update
• Other (please describe)

Does this PR introduce a breaking change? (check one)

• No
• Yes (please describe since breaking changes require a minor version update)

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
• It should address only one issue or feature. If adding multiple features or fixing a bug and adding a new feature, break them into separate PRs if at all possible.
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, chore(docs): fix typo in README, etc.). This is very important, as the CHANGELOG is generated from these messages, and determines the next version type (patch or minor).

If new features/enhancement/fixes are added or changed:

• Includes documentation updates
• Includes component package.json meta section updates (prop, slot and event changes/updates)
• Includes any needed TypeScript declaration file updates
• New/updated tests are included and passing (required for new features and enhancements)
• Existing test suites are passing
• CodeCov for patch has met target (all changes/updates have been tested)
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (Does it affect screen reader users or keyboard only users? Clickable items should be in the tab index, etc.)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)
  • I think it's convincing 🙂. This wasn't a hugely time consuming change so not a big deal if this feature doesn't get approved

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Codecov Report

Merging #6339 (a31f67f) into dev (2a449ab) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev     #6339   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files          299       299           
  Lines        10105     10105           
  Branches      2512      2512           
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  Hits         10105     10105           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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Impacted Files	Coverage Δ
src/components/dropdown/dropdown.js	100.00% <ø> (ø)

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[jacobmllr95]

Thanks @phegman!

[phegman]

@jackmu95 Thanks for reviewing and making those stylistic changes! Bootstrap Vue is an impressively consistent codebase, makes it very enjoyable to work on!