Allow setting of JWT Issuer port

[DomAyre]

Motivation

Since #266, the test JWT issuer doesn't use a fixed port, privacy-sandbox-dev assumes it does which breaks it's testing.
To fix this the options are to either:

• Allow setting the port of the JWT issuer to the port assumed by privacy-sandbox-dev
• Read the full address in privacy-sandbox-dev

Since privacy-sandbox-dev assumes the port in many places (including in an instance running in ACI) it's non-trivial to do the first change, so opting for the latter.

Changes

• Update test JWT Issuer implementation to use the value of JWT_ISSUER_PORT as the port if set (default to 0 if unset)
• Pass that env variable into the JWT issuer container image in docker compose
• Set that value to 3000 for the standalone image used by privacy-sandbox-dev

[DomAyre]

privacy-sandbox-dev tests
https://github.com/microsoft/privacy-sandbox-dev/actions/runs/12689008959