Allow for custom Postgres operators to be added

[boris-glumpler]

In Postgres custom operators can be defined. Currently there is no way, tho, to add these to the grammar. This PR addresses this issue.

For my particular use-case, I am maintaining a package that integrates the pg_search extension by ParadeDB into Laravel. This extension comes with a custom @@@ operator that allows for full-text search. Up until recently, using this operator had some drawbacks and there was another way to integrate the extension, which I initially used for my integration. The use of this operator has been greatly improved in the latest version, tho, and the alternative way has therefore been deprecated.

I have added a static PostgresGrammar::customOperators() method that can be called in service providers to add any needed operators. I looked into adding this to some of the other grammars as well, but couldn't find much information, tbh, so I asked @tpetry about this and apparently Postgres is the only database that allows custom operators.

[boris-glumpler]

pgvector would be another example of a popular extension that adds custom operators that are not in the default set of operators. In this case these would be:

• <-> - L2 distance
• <#> - (negative) inner product
• <=> - cosine distance
• <+> - L1 distance
• <~> - Hamming distance
• <%> - Jaccard distance

[boris-glumpler]

Thanks Taylor!