Add validation that query and table cannot be specified at the same time for SpannerIO.read()

[Amar3tto]

Fixes #24956

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[Amar3tto]

R: @nielm

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[nielm]

Lgtm

[nielm]

R: @chamikaramj for label io

[chamikaramj]

Thanks. LGTM.