feat(b-dropdown-form): add formClass prop for adding classes to the form element (closes #4474)

[bbugh]

Describe the PR

We use b-dropdown-form in a bunch of places (mostly advanced search filters), and want to have control over the form class[es]. It's less complex and more reusable to specifically list the form classes in the parent component, rather than specifying prop options for layout and/or using multiple css selectors. Currently, passing a class prop on b-dropdown-form sets the class on the li.

Closes #4474

PR checklist

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

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe)

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

• No
• Yes (please describe)

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.

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

• Includes documentation updates (including updating the component's package.json for slot and event changes)
• Includes any needed TypeScript declaration file updates
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• CodeCov for patch has met target
• 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)

[codecov]

Codecov Report

Merging #4475 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #4475   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files         247      247           
  Lines        4850     4850           
  Branches     1359     1359           
=======================================
  Hits         4846     4846           
  Misses          3        3           
  Partials        1        1

Impacted Files	Coverage Δ
src/components/dropdown/dropdown-form.js	100% <ø> (ø)	⬆️

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