add conversationStartProps to createDirectLineAppServiceExtension

[stevengum]

Do not merge - Pending Service-side changes & https://github.com/microsoft/BotFramework-DirectLineJS/pull/350

Changelog Entry

Pending

Description

Add support for conversationStartProperties via https://github.com/microsoft/BotFramework-DirectLineJS/pull/350

• [ ] Ensure options bag member name reflects what's final name in DLJS#350

Specific Changes

Scenario example

          window.WebChat.renderWebChat(
              {
                  directLine: await window.WebChat.createDirectLineAppServiceExtension({
                      domain: '<WindowsAppServiceUrlForBot>/.bot/v3/directline',
                      token: response.token,
                      conversationStartProperties: {
                          user: {
                              id: userID,
                              name: username
                          },
                          locale: 'en-us' 
                      }
                  }),
                  store,
                  userID,
                  username
              },
              document.getElementById('webchat')
          );

-

• [ ] I have added tests and executed them locally
• [ ] I have updated CHANGELOG.md
• [ ] I have updated documentation

Review Checklist

This section is for contributors to review your work.

• [ ] Accessibility reviewed (tab order, content readability, alt text, color contrast)
• [ ] Browser and platform compatibilities reviewed
• [ ] CSS styles reviewed (minimal rules, no z-index)
• [ ] Documents reviewed (docs, samples, live demo)
• [ ] Internationalization reviewed (strings, unit formatting)
• [ ] package.json and package-lock.json reviewed
• [ ] Security reviewed (no data URIs, check for nonce leak)
• [ ] Tests reviewed (coverage, legitimacy)