META: Introduce Builder-based configuration system for hardware devices (with optional serde support)

[dclause]

Motivation:
The current device instantiation API (e.g. Button::new_inverted_pullup, etc.) pis ergonomic but not scalable as the number of options grows. It’s also tightly coupled to runtime code and doesn’t support config-based instantiation (TOML/YAML/JSON), which limits flexibility for many real-world use cases.

We propose to replace the current serde support (tied to live device instances) with Builder-based configuration objects for all hardware devices. Builders can optionally be made serializable via a feature-flagged "serde".

Expectation:

• All current serde-related serialization/deserialization is removed from runtime types.
• All devices (and appropriate hardware structures) expose a Builder struct for construction.
• An optional feature "serde" allows serialization/deserialization of these builders.
• Builders may be renamed to Config when exposed via serde to make their intent clearer.

Proposal
Introduce per-device Builder types (possibly named Config when exposed via serde), e.g. ButtonBuilder. These will:

• Be used to construct devices fluently in Rust code.
• Be optionally serializable/deserializable via serde (with a feature flag).
• Completely replace the existing serde implementation, which currently exposes live device types to serialization.

Example

let button = ButtonBuilder::new()
    .pin(2)
    .pullup()
    .inverted()
    .debounce(20)
    .build(&board)?;

Or from config file (enabled with serde):

[[buttons]]
pin = 2
pull = "up"
inverted = true
debounce = 20

Parsed all devices and built:

#[cfg(feature = "serde")]
let config: DeviceConfig= toml::from_str(...)?;
config.buttons.into_iter().try_for_each(|b| b.build(&board))?;