Simplify reading Connect pins as the viewer #363

#362 documents how to read pins as the viewer on Posit Connect with a visitor API key. The example must read the Posit-Connect-User-Session-Token header from the Shiny session and exchange it with posit-sdk:

token = session.http_conn.headers.get("Posit-Connect-User-Session-Token")
client = connect.Client()
credentials = client.oauth.get_credentials(
    token,
    requested_token_type="urn:posit:connect:api-key",
)
board = board_connect(server_url=..., api_key=credentials["access_token"], cache=None)

Users must know about the header, the token exchange, and the token type, and it is easy to get wrong. Client.with_user_session_token() already does the exchange, but it does not expose the API key that pins needs.

In the review of the matching pins-r docs (rstudio/pins-r#906), a helper was proposed, so that the example does not need to explain the header or the exchange.

Tasks:

• Add a helper to posit-sdk that takes a Shiny session and returns the visitor API key, or a client that exposes it.
• Update the visitor example in docs/index.qmd to use the helper.
• Decide if board_connect() should accept the visitor client or session directly, so that users do not pass the key and cache by hand.