sql, opt: avoid full scans in mutation queries

[rytaft]

opt,sql: support hint to avoid full scan

Informs #79683

Release note (sql change): Added support for a new index hint,
AVOID_FULL_SCAN, which will prevent the optimizer from planning a
full scan for the specified table if any other plan is possible. The
hint can be used in the same way as other existing index hints. For
example, SELECT * FROM table_name@{AVOID_FULL_SCAN};. Note that a
full scan of a partial index may still be preferred by the optimizer,
unless AVOID_FULL_SCAN is used in combination with a specific partial
index via FORCE_INDEX=index_name. This hint is similar to NO_FULL_SCAN,
but will not error if a full scan cannot be avoided.

sql,opt: add setting avoid_full_table_scans_in_mutations

Fixes #79683

Release note (sql change): Added a new session setting
avoid_full_table_scans_in_mutations, which when set to true, causes
the optimizer to avoid planning full table scans for mutation queries
if any other plan is possible. It now defaults to true.

opt: remove a stale comment above optbuilder.buildScan

Removed a comment that references a function parameter that no longer
exists.

Release note: None

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This change is 

[rytaft]

Reviewable status: complete! 0 of 0 LGTMs obtained

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pkg/sql/opt/exec/execbuilder/testdata/update line 404 at r3 (raw file):

└── • render
    │
    └── • limit

Need to investigate why we lost the top k here

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pkg/sql/opt/exec/execbuilder/testdata/unique line 5707 at r3 (raw file):

vectorized: true
·
• root

This seems concerning that we're no longer getting the insert fast path here.