feat(popover/tooltip): Add boundaryPadding prop to override Popper.js default padding

[jacobmllr95]

Description of Pull Request:

This PR add a new boundaryPadding property for <b-popover> and <b-tooltip> components to override Popper.js default padding to the boundariesElement of 5px.

PR checklist:

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

• Bugfix
• Feature
• Enhancement to an existing feature
• ARIA accessibility
• Documentation update
• Other, please describe:

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

• Yes
• No

If yes, please describe the impact:

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 #xxxx[,#xxxx], where "xxxx" is the issue number)
• The PR 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.
• PR titles should following the
  Conventional Commits naming convention (i.e.
  "fix(alert): not alerting during SSR render", "docs(badge): Updated pill examples, fix typos",
  "chore: fix typo in docs", 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)
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• 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
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[codecov]

Codecov Report

Merging #2475 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #2475   +/-   ##
=======================================
  Coverage   71.63%   71.63%           
=======================================
  Files         172      172           
  Lines        3240     3240           
  Branches      916      916           
=======================================
  Hits         2321     2321           
  Misses        667      667           
  Partials      252      252

Impacted Files	Coverage Δ
src/utils/tooltip.class.js	100% <ø> (ø)	⬆️
src/mixins/toolpop.js	42.66% <ø> (ø)	⬆️

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[tmorehouse]

@jackmu95 could you maybe add a small blurb to the docs about the new prop?

[jacobmllr95]

@jackmu95 could you maybe add a small blurb to the docs about the new prop?

Done.