Improve CLI and .env naming of HASURA_GRAPHQL_ENDPOINT

Wherever the CLI takes "endpoint" arguments like `--endpoint` or the `HASURA_GRAPHQL_ENDPOINT` in `.env` it would be nice to somehow make it clear that the endpoint is the Hasura graphql engine endpoint without the `v1/graphql` piece.

Some alternative ideas:
1. perhaps we just build in an improvement where the CLI can try again by trying to strip the `v1/graphql` endpoint?
2. Improve our endpoint error messages everywhere to suggest that this should just be endpoint without the graphql piece, or point to docs on how to construct the endpoint

This would prevent errors like this:

```
$ hasura metadata export --endpoint https://localhost:8080/v1/graphql

FATA[0001] {
"path": "$",
"error": "resource does not exist",
"code": "not-found"
}

```

Just a minor thing but would go a long way in DX delight <3

Start by tracing the CLI paths that accept --endpoint and the .env handling for HASURA_GRAPHQL_ENDPOINT. Compare how endpoint errors are produced across the affected commands, then clarify with maintainers which proposed behavior is wanted. Done should include an agreed endpoint convention and consistent guidance or handling in the relevant paths.

Written by the indexing model from the issue text.