[UDFs] Ability to specify a UDF type

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

Right now, we are only supporting "scalar" (UDF) functions. However, there are also aggregate (UDAF) and table (UDTF). We should allow the specification of a function type so that these (and perhaps future new types?) can be supported.

Acceptance criteria

• The YAML definition of a function accepts a type value

Suggested Tests

• Specifying the type property of a function in YAML properly propagates to the node

Impact to Other Teams

N/A

Will backports be required?

No

Context

No response

[FishtownBuildBot]

Opened a new issue in dbt-labs/docs.getdbt.com: dbt-labs/docs.getdbt.com#7958