Add per-index "shrink" option to control record-growing UPDATE optimization

MDEV-38814 introduces a new global variable innodb_index_shrink (opt-in) to control whether record-growing UPDATE should aggressively attempt shrinking the B-tree resorting to pessimistic path (legacy behavior), or can allow optimistic path more often (faster, less contended), at cost of sligthly sparser trees (new behavior).
The default for the variable, designed for 11.8 LTS release, is ON (legacy, opt-in).
For main, since MDEV-37070 is merged, we can introduce a per-index shrink option to allow granular opt-in for the feature, since it will benefit only those indices touched by those record-growing UPDATE (for example NULL to not-NULL).
In this scope, innodb_index_shrink can be repurposed as the session variable which states the default shrink value to use in the subsequent DDL statements of the session, as per marko suggestion.