feat: mgt-picker selected-value attribute

[gavinbarron]

Closes #2353

PR Type

• Feature

Description of the changes

Allows for setting the selected value of the picker for scenarios where the application wants to control the selected item

PR checklist

• Project builds (yarn build) and changes have been tested in at least two supported browsers (Edge + non-Chromium based browser)
• All public APIs (classes, methods, etc) have been documented following the jsdoc syntax
• Stories have been added and existing stories have been tested
• Added appropriate documentation. Docs PR: https://github.com/microsoftgraph/microsoft-graph-docs/pull/21481

• License header has been added to all new source files (yarn setLicense)
• Contains NO breaking changes

Other information

[ghost]

Thank you for creating a Pull Request @gavinbarron.

This is a checklist for the PR reviewer(s) to complete before approving and merging this PR:

• I have verified a documentation PR has been linked and is approved (or not applicable)
• I have ran this PR locally and have tested the fix/feature
• I have verified that stories have been added to storybook (or not applicable)
• I have tested existing stories in storybook to verify no regression has occured
• I have tested the solution in at least two browsers (Edge + 1 non-Chromium based browser)