Remove access token length check

[PFacheris]

Description: A recent attempt to integrate this library with lightstep.com requires use of an access token of varying length (encoded as a JWT). Currently this is disallowed by the library due to the strict length requirements on the access token. This PR removes that check.

Link to tracking Issue: N/A

Testing: Fork tested in an application that makes use of the launcher. It is configured via

launcher.ConfigureOpentelemetry(
    launcher.WithServiceName(config.App.Name),
    launcher.WithServiceVersion(config.App.Version),
    launcher.WithMetricsEnabled(false)
)

and the access token is set via LS_ACCESS_TOKEN env var.

Documentation: N/A

[jmacd]

@codeboten and @carlosalberto What do you think? I think customers are interested in using OTel collectors that are capable of adding access tokens on behalf of clients, so we can relax this restriction, yes?

[MadVikingGod]

I think this makes sense. We could also leave it as a warning if the len>0 but not one of the prescribed lengths.