sqlparser: normalize IndexInfo

[vmg]

Description

The IndexInfo AST node in the sqlparser was a bit crusty, particularly with the many boolean fields that could instead be represented as an enum. This refactoring also normalizes the generated SQL expressions when index types are involved, including the lowercasing of all index keywords, which we weren't doing properly before.

cc @GuptaManan100 who advised on this

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[GuptaManan100]

LGTM! Thank-you for making these changes!