feat: allow providing multiple apolloClients outside of setup/vue context in vue-apollo-composable

[lzurbriggen]

What?

• adds a provideApolloClients that mirrors the api of app.provide(ApolloClients, { <clients> }) to allow using different apollo clients outside of setup/vue contexts (for "global queries")
• should also fix the issue in fix: provideApollClient overriding specified query / mutation clientId #1338

Why?

• It was impossible to use global queries/mutations outside of setup

How?

• useApolloClients.ts now maintains a currentApolloClients client dictionary instead of just a single client

I added an extra e2e test and a short section in the docs.

[dcress]

@Akryum Could we get some 👀 on this? This would be VERY helpful for those of us using multiple clients.

[bianpratama]

@Akryum patiently wait for this 🙏🏻

[dcress]

@Akryum 🙏