Implement builder interface for SelectorParams.

[aagbotemi]

Building transactions through SelectorParams currently gets no feedback if they accidentally create dust outputs or include multiple OP_RETURN outputs. These mistakes only surface at relay time with opaque node rejections.
Implement a builder interface for SelectorParams that validates target outputs against configurable mempool standardness rules, cathching dust recipients, multiple OP_RETURN outputs, oversized OP_RETURN scripts, and missing required fields at construction time.

Proposed solution
Introduce a SelectorParamsBuilder as a separate type that validates and produces SelectorParams.
The SelectorParams::builder() chains methods and call .build() which validates against a configurable MempoolPolicy struct.

Context
See selector.rs L20–L27

[aagbotemi]

Update on the design while I've been implementing this. The OP_RETURN rule in the TODO ("error on multi OP_RETURN outputs") is no longer accurate under Bitcoin Core v30 bitcoin/bitcoin#32406

v30 did two things: it removed the single-OP_RETURN restriction entirely, and it changed -datacarriersize from a per-output cap to a budget spread across all OP_RETURN outputs in a transaction. The default was raised to 100,000 bytes, which effectively uncaps at default settings.
That leaves a policy choice for check_standardness:

1. Match v30 defaults - 100,000 bytes.
2. Match the pre-v30 - 83 bytes.
3. Make it configurable on SelectorParams.

I've implemented option 1 so far.