feat: make Spanner SQL DDL parsing public

[gurminder71]

The Spanner SQL statements are specific to Spanner database. The DDLDiff that parses the DDL statement and converts to in-memory AST tree is valuable to be used outside of the diff generation.

Other Java programs can use the parsed information for other usages like CI testing on the structure of the schema

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
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