Refactor SCIM ParamsParser

What

• Scim::ParamsParser processes params based on request instead of guessing by contents.
• Extracts value coercion/casting to a new ValueParser class.
• Extracts attribute mapping to a new AtrributeTransform class.
• Extracts GET filter parsing to a new FilterParser class

Why

• Previously we only processed params for GET /Users if params[:filter] was present, and would have attempted to look at POST params if it was not.
• This represents most of the work for #35209 (closed), with a further small MR to create the actual behaviour change.
• Properly supporting the SCIM filtering spec for #13138 (closed) will require us to write an actual parser, likely with parslet. Splitting out these classes reduces the interdependencies between them in preparation for that effort.

Acceptance criteria

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.