Use Continue/version as the user-agent in customFetch

[jhrozek]

Description

When continue is frontent by a proxy (such as codegate), the proxy
benefits from knowing who it is that is talking to it instead of a
generic JS user-agent.

This patch enhances the requestOptions with a custom header set to
Continue+version.

Checklist

• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screenshots

N/A

Testing instructions

N/A

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[jhrozek]

Oops sorry:

    SyntaxError: The requested module '../package.json' does not provide an export named 'version'

      at Runtime.linkAndEvaluateModule (node_modules/jest-runtime/build/index.js:708:5)

I gotta look at this, I wonder why the code worked locally for me when I ran the extension.

[jhrozek]

The checks that were failing earlier are now passing. I'm not sure if the tests that are failing now are related to my changes and it seems that I lack the permissions to re-run them.

[jhrozek]

resolved conflicts, no other changes

[jhrozek]

@sestinj is this something that continue would be interested in merging? Only after opening this PR did I notice PR #2988 so the answer might very well be "no" - that's fine, but in that case, would Continue be interested in merging a custom header (e.g. x-coding-assistant: continue) ?