[url_launcher] Add support for setting show title on Chrome Custom Tabs

[Alex-Usmanov]

• Adds showTitle option to platform interface & app-facing api
• Makes use of that parameter on android platform in order to set show title on Chrome Custom Tabs, when it is being used
• Makes android implementation use both legacy & new api (instead of passthrough from new to legacy), in order to allow use of the aforementioned parameter
• Adds a button to the example to test this parameter
• Adds tests

(Notice title at the top of the page "CyLog Software - HTTP ...")

https://github.com/flutter/flutter/issues/136784

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [x] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [ ] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[Alex-Usmanov]

I made a terrible mistake with source control and #5166 was automatically closed. This basically #5166 in the latest condition it ever was.

I am so sorry for the inconvenience. This PR was reviewed and is in progress of landing.

Platform interface portion: #5758 Platform implementation portion: #5759

[Alex-Usmanov]

This is blocked on #5759, which is currently in review.

[stuartmorgan-g]

@Alex-Usmanov Is this ready for re-review? The blocking PR has landed.

[Alex-Usmanov]

Yes, sir. It is ready for re-review