feat(bigquery): add anonymous connection method using AnonymousCredentials

[luispedroemg]

resolves dbt-labs/dbt-bigquery#358
docs dbt-labs/docs.getdbt.com/#

Related PR: #1045

Note: Even though issue dbt-labs/dbt-bigquery#358 has been marked as resolved, it is only partially resolved. PR #1045 added the api_endpoint config, but in order to use (at least some) of the bigquery emulators, we need to be able to use Anonymous credentials.

Problem

BigQuery users need a way to connect without credentials (e.g., BigQuery emulator or public/read-only endpoints). Today, dbt-bigquery requires an authenticated method, which blocks these scenarios.

Solution

Add a new connection method, ANONYMOUS, to BigQueryConnectionMethod.
When method=anonymous, create google.auth.credentials.AnonymousCredentials.
Add a unit test that validates the anonymous config and connection acquisition.
No changes to default behavior; this is opt-in and limited to the BigQuery adapter.

This in conjunction with #1045, will allow users to setup dbt to connect to an emulator like https://github.com/goccy/bigquery-emulator

Alternatives Considered

• Implicitly falling back to anonymous when no credentials are provided (risk of surprising behavior).
• Adding flags to oauth or service-account methods (less clear than an explicit method).

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

Note: This introduces a new profile method value: "anonymous"

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