Allow custom headers to be set on JWKClient requests

[thundercat1]

When using PyJWKClient to access a jwks server behind a firewall, I've run into problems with my request being denied due to the default Python-urllib/3.9 user agent.

Would the maintainers entertain allowing users to specify a custom headers dictionary when creating the pyJWKClient that would automatically be set on fetch requests to retrieve the JWK? By allowing a generic header dictionary to be included, you could solve not only the user agent difficulties I've run into, but other use cases as well.

Thank you!

[github-actions]

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