feat(trino): support per-query user impersonation

[nakulgan]

Description

Adds opt-in per-query user impersonation to the trino-sql and
trino-execute-sql tools.

When impersonateUser: true is set on the tool, it exposes an additional
optional trino_user input parameter. Its value is forwarded as the
X-Trino-User header for that statement only, so a single pooled connection can
run individual queries as different users. If trino_user is omitted or empty,
the query runs as the source's configured user — so existing configs are
unaffected (the field defaults to false).

Motivation / impact. Trino resource groups and access control are commonly
keyed on the request user. Without impersonation, every query a Toolbox instance
issues is attributed to one static identity, so they all share a single resource
group's concurrency/queue limits and lose per-user attribution. Forwarding the
end user lets each query carry that identity for scheduling isolation and
auditing.

Solution. The source gains a RunSQLAsUser method that attaches the user as
a sql.Named("X-Trino-User", ...) query argument. The trino-go-client
forwards any X-Trino--prefixed argument as a request header and excludes it
from positional ? placeholder binding, so the impersonation user never
consumes a parameter slot. The pool's configured principal still authenticates
the request, so it must be authorized to impersonate on the Trino side.

PR Checklist

• Make sure you reviewed CONTRIBUTING.md
• Make sure to open an issue as a bug/issue before writing your code — happy to file one if the maintainers prefer to discuss the design first
• Ensure the tests and linter pass
• Code coverage does not decrease — added unit tests for the routing in both tools and an integration test asserting current_user reflects the impersonated user
• Appropriate docs were updated
• Make sure to add ! if this involves a breaking change — not applicable; the field is opt-in and defaults to false

🛠️ Fixes #

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[gemini-code-assist]

Code Review

This pull request introduces user impersonation support for the Trino 'trino-execute-sql' and 'trino-sql' tools by adding an optional 'impersonateUser' configuration option and forwarding the 'trino_user' parameter as the 'X-Trino-User' header. Feedback suggests refactoring the test 'mockSource' structs in both 'trinoexecutesql_test.go' and 'trinosql_test.go' to implement the 'sources.Source' interface methods directly rather than embedding the interface, which can lead to nil pointer dereferences if uninitialized.