feat: add user-agent to headers

[kobenguyent]

if user doesn't specify the user-agent header, restfox would add it to the request to help the server identify Restfox as the HTTP requesting application or client.

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[flawiddsouza]

Hi. Thanks for the PR. However there are some issues.

1. This will hardcode the user agent to Restfox + version that the user is using at the time of creating the request.
2. We are adding header data that is non essential to the user into user's data (only required for IDing the client)
3. Right click request -> Copy to CURL now includes the user agent in it, which is not desirable for the user

To rectify this, we can do this:

• We will show the default headers above the usual headers in a table. These headers (currently only one) will be unmodifiable (can't uncheck, can't delete) [similar to how its done in other popular http clients]
• We will have a small note below the default headers table saying "These headers can be overriden below if required."
• The default header will be dynamic and will always send the actual version of restfox that is currently in use - this should be implemented in handleRequest method in the helpers.ts file + we should not add the default header User-Agent if user has an enabled header that is User-Agent.

[flawiddsouza]

On second thought, maybe we don't need to show the default headers in the headers section. Just adding them directly to the request when sending it should be enough. Just implementing the 3rd point should be good enough.

[kobenguyent]

@flawiddsouza thanks for your review! Addressed as your proposal!