Create interceptor for processing custom tagged template value expressions

[gajus]

The way that Slonik is using tagged template literals to generate code fragments and safely bind parameter values is particularly exciting. Therefore, I am exploring feasibility of allowing community to bring their own code-generating extensions that integrate into Slonik template language. These extensions will allow to abstract business-specific fragments of code (e.g. dynamic WHERE conditions for a particular business requirement); big development teams can use this pattern to restrict any use of raw.sql in the main codebase. This enables separating integration and code generation tests.

The primary considerations:

• Design of value expression that does not compromise the strict type nature of the current tags.
• Client configuration that restricts use of sql.raw tokens, but allows custom extensions.

[gajus]

This should also provide features equivalent to https://github.com/vitaly-t/pg-promise#custom-type-formatting.

[gajus]

Users can already create custom tags simply by using the already available query building utilities.