add support for RTREE index

[leiysky]

What problem does this PR solve?

Add support for RTREE index.

What is changed and how it works?

MySQL syntax:

index_type:
          BTREE_SYM { $$= HA_KEY_ALG_BTREE; }
        | RTREE_SYM { $$= HA_KEY_ALG_RTREE; }
        | HASH_SYM  { $$= HA_KEY_ALG_HASH; }
        ;

Check List

Tests

• Unit test

[codecov]

Codecov Report

Merging #433 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
+ Coverage   70.83%   70.84%   +<.01%     
==========================================
  Files          32       32              
  Lines        7499     7501       +2     
==========================================
+ Hits         5312     5314       +2     
  Misses       1674     1674              
  Partials      513      513

Impacted Files	Coverage Δ
parser.go	70.58% <ø> (ø)	⬆️
misc.go	96.42% <ø> (ø)	⬆️
model/model.go	48.93% <100%> (+0.36%)	⬆️

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[zier-one]

LGTM

[zier-one]

@kennytm PTAL

[tangenta]

LGTM