sql: Add information_schema.session_variables table

[mneverov]

sql: Add information_schema.session_variables table

The session_variables table exposes the session variables.
Related to: #8675

Release note (sql change): Added session_variables table to the information_schema.

[blathers-crl]

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I have added a few people who may be able to assist in reviewing:

• @jordanlewis (commented on sql: Complete the information_schema by adding empty table definitions #8675)
• @rafiss (commented on sql: Complete the information_schema by adding empty table definitions #8675)
• @nvanbenschoten (author of sql: Complete the information_schema by adding empty table definitions #8675)
• @otan (commented on sql: Complete the information_schema by adding empty table definitions #8675)

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[cockroach-teamcity]

This change is 

[mneverov]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @jordanlewis, @nvanbenschoten, @otan, and @rafiss)

a discussion (no related file):
To the reviewer: it appears CRDB does already contain this information in crdb_internal.session_variables.
This commit is just a copy-paste from the crdb_internal.session_variables without hidden column.
Please let me know if hidden variables should be filtered out: allow_prepare_as_opt_plan, save_tables_prefix, session_authorization, ssl_renegotiation_limit

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[blathers-crl]

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[rafiss]

thanks for your contribution!

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)