functions/spanner cleanup

[leahecole]

based on suggestions from @grayside and @dandhlee

• move functions/spanner code to spanner/functions (or possibly to the spanner repo - can't remember where spanner samples live right now)
• update /spanner/* in codeowners to @GoogleCloudPlatform/api-spanner-python

[iennae]

Adding spanner to make sure we get folks aware

[kweinmeister]

We have already updated CODEOWNERS to point to the Spanner team.

Do we still plan to move these samples within the repo anytime soon (along with doc references)? If not, I'd like to suggest closing this 2 year old issue.

@iennae @engelke @grayside