fix: prevent override of client request listeners in CodeWhispererServiceIAM

[francescoopiccoli]

Problem

Prevent override of client request listeners in CodeWhispererServiceIAM in case any listener was setup before.

Solution

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[viktorsaws]

Do we need to do this with caching original listeners? Would using similar implementation as for CodeWhispererServiceToken work

language-servers/server/aws-lsp-codewhisperer/src/language-server/codeWhispererService.ts

Lines 145 to 154 in aad4188

	onRequestSetup: [
	req => {
	req.on('build', ({ httpRequest }) => {
	const creds = credentialsProvider.getCredentials('bearer') as BearerCredentials
	if (!creds?.token) {
	throw new Error('Authorization failed, bearer token is not set')
	}
	httpRequest.headers['Authorization'] = `Bearer ${creds.token}`
	httpRequest.headers['x-amzn-codewhisperer-optout'] = `${!this.shareCodeWhispererContentWithAWS}`
	})

?

[francescoopiccoli]

I have not tested but I am confident that it would work as well. I am fine with going with that approach if you think it's better, but I would be curious to know why you think it is better, apart from consistency with the the token client implementation.
I did consider that approach before opening the PR, but it felt like it was adding more complexity as you need to extend the options (see here), and you still do cache the original listeners from my understanding (see here)