Har importer

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PR Description

Har importer implementation

Related Issues

• Closes Importing Requests from HAR file #122

Checklist

• I have gone through the contributing guide
• [x ] I have updated my branch and synced it with project main branch before making this PR
• [x ] I am using the latest Flutter stable branch (run flutter upgrade and verify)
• [x ] I have run the tests (flutter test) and all tests are passing

Added/updated tests?

packages/har_parser/test

• Yes
• No, and this is why: please replace this line with details on why tests have not been included

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux

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@animator @ashitaprasad
This PR has been open for a while. This solves the high priority feature #122 and is inline with the apidash design and coding standards. Request your review. Please let me know what more needs to be done to get this merge ready. Will be happy to work on it on priority and close.

[animator]

We are currently busy with GSoC application reviews. This will be reviewed post that.