Struct type should be a segment

[alanmcruickshank]

Was initially part of #3428 but to required as part of new fix #3590.

I think this still makes sense - the other Struct-like things (e.g. TypelessStructSegment) are segments and StructTypeGrammar was the odd one out. This swaps StructTypeGrammar to instead be StructTypeSegment.

This also removes the unnecessary and unreferenced definition of StructKeywordSegment.

[codecov]

Codecov Report

Merging #3591 (445e76d) into main (f49122d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #3591   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          174       174           
  Lines        13206     13211    +5     
=========================================
+ Hits         13206     13211    +5     

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_ansi.py	100.00% <100.00%> (ø)
src/sqlfluff/dialects/dialect_bigquery.py	100.00% <100.00%> (ø)

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