Add optional Fields parameter to log query functions

[tustanivsky]

This PR adds an optional Fields parameter to Get-SentryLogsByAttribute and Get-SentryTestLog functions that allows callers to request additional log-related fields in the API response.

Motivation: Integration tests for structured logging need to assert on custom log attributes beyond the default field set. Previously, the field list was hardcoded and there was no way to include additional attributes in the response without modifying the module itself.

Example usage:

# Request additional fields in the response
$logs = Get-SentryTestLog -AttributeName 'test_id' -AttributeValue $testId `
    -Fields @('sentry.environment', 'custom_attr')

# Fields are available as properties on returned log objects
$logs[0].'sentry.environment' | Should -Be 'production'