THREESCALE-10688: Enable `Content-Security-Policy`

[jlledom]

What this PR does / why we need it:

Running Rapidast against 3scale reveals the next warning about apisonator:

Content Security Policy (CSP) Header Not Set

In order to fix the warning, we must return the CSP header in all requests to the listener

Which issue(s) this PR fixes

https://issues.redhat.com/browse/THREESCALE-10688

Verification steps

Curl the listener with the appropriate parameters:

curl -v 'http://localhost:3001/transactions/authrep.xml?service_token=<TOKEN>&service_id=<SERVICE_ID>&user_key=<USER_KEY>&usage%5Bhits%5D=1'

You should see the CSP header in the response:

content-security-policy: default-src 'self'

Special notes for your reviewer:

Some useful links about CSP:

• https://content-security-policy.com/
• https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/Sources