Add URI::Params::Field ignore options

[RoySerbi]

Resolves #16873.

This adds the same basic field-skipping options to URI::Params::Field that are already available on JSON::Field and YAML::Field:

• ignore: true skips both serialization and deserialization
• ignore_serialize: true skips #to_www_form
• ignore_deserialize: true skips .from_www_form

I also updated the URI::Params::Serializable docs and added specs covering each case.

Local check:

• git diff --check

I could not run the Crystal specs locally because this Windows environment does not have the Crystal compiler installed.

[straight-shoota]

There is already #17029 pending for the same feature and the implementation looks better: #16873 (comment)

[RoySerbi]

@straight-shoota Good catch on the parity with #17029 — looking at src/json/serialization.cr, JSON::Serializable's ignore_serialize accepts any Crystal expression and evaluates it at runtime (unless {{ value[:ignore_serialize] }}), which is exactly what #17029 mirrored and what this PR was missing.

7cd19a6 brings this PR to the same convention:

• Literal ignore_serialize: true is still elided at compile time (no codegen change for the common case).
• Any other expression is emitted as unless {{ expr }} around the field's to_www_form call, so ignore_serialize: internal (or any boolean expression over instance state) works at runtime.
• Docstring updated to match JSON::Field's wording verbatim.
• Added a spec exercising the runtime-expression case (truthy skips, falsy emits).

The deserialization side is unchanged (still compile-time only for ignore / ignore_deserialize), matching JSON::Serializable's asymmetry. Happy to fold in trypsynth's spec style or any other tweaks if you'd prefer this over #17029.