Ignore adapter-level support for custom constraints

[jtcohen6]

resolves #90

I believe that fixing this bug would enable a workaround for defining tags / masking policies as part of Snowflake model contracts. This has recently come up as a blocking issue for multiple folks internally (see internal slack thread).

Problem

ConstraintType.custom is missing from CONSTRAINT_SUPPORT, leading to a KeyError during process_parsed_constraint.

Solution

Completely ignore "custom" constraints when checking for adapter-specific support in process_parsed_constraint.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

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