Standardize on the way to define the spector project

[timotheeguerin]

Seems like there is quite a few ways to run the spector tests, while some thing probably need to vary for the various emitters, the base should still be the same. Java for example uses currently a separate package.json which requires an npm install which is not super clean

[msyyc]

python follow pnpm install in ci-python.yml to align with typespec-azure repo

[XiaofeiCao]

java's plan to unify e2e test run: #5008