Support setting up proxy via provider attribute `proxy_url`

[take-five]

Previously, the only way to configure proxying requests to Postgres through a SOCKS5 proxy server was to set ALL_PROXY environment variable. However, this could have unintended side effects and impact other providers.

With proxy_url provider attribute, users can configure proxy for each provider instance individually and make use of e.g. Terraform variables.

It is also possible to use PGPROXY environment variable to configure SOCKS5 proxy for all instances of PostgreSQL provider in the current root module. This environment variable is also used in acceptance tests for this provider.

[esulzickis]

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