implemented har feature

[hrajwade96]

• Export har feature added.
• Need some guidance regarding deserialising the har and offline support.

List which issues are fixed by this PR.

Please add a note to packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md if your change requires release notes. Otherwise, add the 'release-notes-not-required' label to the PR.

Pre-launch Checklist

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [ ] I read the Flutter Style Guide recently, and have followed its advice.
• [ ] I signed the CLA.
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or there is a reason for not adding tests.

If you need help, consider asking for help on Discord.

[hrajwade96]

@kenzieschmoll @exaby73 When I disconnect the app loader keeps showing https://github.com/flutter/devtools/issues/7968