feat(spanner): Support directed read options

[NivedhaSenthil]

• Support directed_read_options at client creation
• Pass on the options to session when session is created from client
• Support directed_read_options at per request level for streaming_reads and streaming_query_execution
• Options set at request level takes precedence at the session
• Add units to assert the option passing and precedence
• Add acceptance test to test one happy path passing directed_read_options

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closes: #<issue_number_goes_here>

[SandeepTuniki]

Suggested minor changes for improvement. But otherwise, LGTM.

[harshachinta]

LGTM. Please fix failing builds.
Where do we have the samples for this feature?

[NivedhaSenthil]

Benchmarking failure is unrelated to the changes, it has to be handled separately. Samples PR is in another repo GoogleCloudPlatform/ruby-docs-samples#1193